P189 Background: Breastfeeding is an important route of HTLV transmission. Risk increases in later breastfeeding. HIV-1 transmission also occurs by breast feeding but has been inadequately studied. This study, conducted in Malawi, Africa between 1994 and 1998, is the largest yet reported. Objective: To understand the risks of HIV transmission from breastmilk and examine possible risk factors that might identity subgroups at high risk. Methods: Subjects were enrolled if born to HIV-1 infected women, still breastfeeding and uninfected at the time of the first postnatal visit. These initially uninfected infants were enrolled until they stopped breast-feeding, were lost to follow-up, or became infected. Infection was shown by PCR. Midpoint between last negative and first positive samples was used as the time of infection. Risk factors were analyzed as relative risks (RR) from proportional hazards models. A second study examined the correlation of transmission with mastitis (as defined by high breast milk sodium). Results: We enrolled 672 mother-infant pairs Breastfeeding continued for a median of 22 months (25%-75%: 16-25 months). Transmission occurred at a rate of at least 1.0, 0.6, 0.4 and 0.4 percent per month for the first 4 semesters of life (ptrend 0.01). We examined mother's age, parity, delivery mode, birth weight and clinical mastitis, but none of these risk factors identified women more likely to transmit HIV by breast-feeding. However, in the second study, we were able to correlate HIV-1 transmission with high viral levels and also with high sodium levels in breast milk. Infants breastfeeding 24 month has a risk of at least 14.8%. Conclusions: In contrast to HTLV, transmission of HIV is more common in the early months of breastfeeding. Since we were not able to examine transmission in colostrum or first month feeding, the cumulative risk we observed may underestimate transmission rates. While we could not identify a subgroup at high risk on the basis of questionnaire data, women who transmitted were likely to have high viral loads. They also had high sodium levels in breast milk, suggestive of subclinical mastitis.