Few published data are available on the prevalence of HIV infection among transsexual individuals and, to our knowledge, no data are available on the incidence of infection in this population. Moreover, most studies examining HIV seroprevalence have focused exclusively on male-to-female (MtF) transsexuals who engage in sex work [1,2]. We report the results of a study that assessed HIV seroprevalence and seroincidence among MtF transsexuals receiving HIV prevention services in Los Angeles County, USA. From February 1998 to January 1999, 244 individuals who were 18 years or older, resided in Los Angeles County, and identified as MtF transsexual or as a woman who was born male were enrolled in the study. Biological men who dressed as women but identified as men (e.g. transvestites) were not included. Participants were recruited at three community agencies and in outreach settings where staff from the agencies were delivering HIV prevention services. These services ranged from brief one-time encounters to structured prevention interventions involving multiple contacts. Each participant received a baseline interview and an oral fluid-based HIV antibody test. A follow-up interview and HIV antibody test were scheduled for 6 months after the initial interview. Interviews were conducted in English, Spanish, and Tagalog. The racial/ethnic mix of the study group was 49% Latino, 20% Asian/Pacific Islander, 15% white, 7% black, and 8% multi-racial or other (Table 1). The median age was 29 years (range 18–61 years). Approximately one half reported less than 12 years of education (47%) and an annual income of less than US$12 000 (50%). Nine per cent reported homelessness in the past 6 months. Fifty-eight per cent reported having ever been incarcerated and 50% reported sex work as a main source of income.Table 1: HIV seroprevalence by sociodemographic characteristic among male-to-female transsexuals. More than half (56%) identified their gender as female or woman, and most (77%) reported their sexual orientation as heterosexual. Eighty-five per cent reported sexual contact with a man in the past 6 months. Thirty per cent had previously undergone surgery to enhance their gender presentation, most commonly breast augmentation (21%), but only 3% had undergone genital surgery for sex reassignment. More than half (64%) reported no health insurance coverage and 32% no regular source of health care. HIV seroprevalence at baseline was 22% (Table 1). Seroprevalence was higher among those aged 30–39 years (40%), those with annual incomes less than US$12 000 (31%), and those with less than 12 years of education (29%). Seroprevalence was 26% among those who reported sex work as a main source of income and 18% among all others (P = 0.11). Although a substantial variation in seroprevalence was also observed by race/ethnicity, these results may have been influenced by differences in recruitment strategies at the three agencies. For example, recruitment at the agency serving a largely Hispanic clientele occurred primarily in outreach settings, where those believed to be at highest HIV risk were receiving prevention services. Follow-up information was obtained on 219 (90%) individuals. The analysis of HIV seroincidence was restricted to those in this group who were HIV-seronegative at baseline (n = 171). Follow-up intervals for these individuals ranged from 5 to 17 months (mean 8 months), producing a total of 117.0 years of person-time follow-up. Four seroconversions were identified in the group, an incidence rate of 3.4 infections per 100 person-years. The prevalence of HIV infection observed in this group of MtF transsexuals is among the highest seen in any group in Los Angeles County over the past two decades, and is comparable to prevalence rates measured among high-risk gay and bisexual men (e.g. those receiving services in public sexually transmitted disease clinics; Los Angeles County Department of Health Services, unpublished data). The observed seroincidence is comparable to rates measured among gay and bisexual men in the mid- to late-1980s [3], and highlights the urgent need for more effective transsexual-specific HIV prevention strategies. Our findings should not be generalized to other MtF transsexual populations, particularly those more affluent and assimilated. In addition, the results may not be representative of transsexual individuals not receiving prevention services who live in or frequent the neighborhoods served by the three participating agencies. The high seroprevalence among African-Americans (44%) should be interpreted with caution given the small sample size (n = 18). This finding, however, is consistent with a recent study in San Francisco [4], which found a seroprevalence of 63% in African-American MtF transsexuals, more than double the rate found in the other racial/ethnic groups studied. Paul A. Simonab Cathy J. Rebackc Cathleen C. Bemisb