Data on the prevalence of social disablement in long-stay psychiatric patients, and the assessment of the psychometric properties of the instruments that evaluate social behaviour in this population are scarce in Brazil. Therefore, this cross-sectional study aimed to estimate the prevalence rates of social disablement in a population of long-stay psychiatric patients from the Rio de Janeiro metropolitan area, and assessed the psychometric properties of the Social Behaviour Schedule (SBS). Data were collected from a population of 881 psychiatric patients housed in the Municipal Mental Health Institute using the 21-item SBS. Most of the patients were women (59%), the mean age was 65.8 years (SD = 11) and the mean length of stay was 37.3 years (SD = 11.5). Of the population, 50.6% were scored as having poor self-care, 46% with little spontaneous communication, 41.1% with poor attention span, and 37.1% with underactivity. Comparing our data with international studies that used the same instrument, we found that our population was more disabled than the others, especially on the social withdrawal factor. Regarding psychometric properties, the inter-rater kappa was 0.709, the inter-informant kappa was 0.500, and the Cronbach's alpha coefficient was 0.766. The groups of patients in the six settings of the institute presented significant statistical differences in the total score (F = 11.447, p < 0.001). This study demonstrates the high rates of social disablement in this population. The precarious conditions of the institution where the patients have been living for decades and unmet individual care may have exacerbated their social disablement. Furthermore, the SBS-BR had satisfactory psychometric properties, particularly reliability, showing it to be an adequate instrument for measuring social disablement in Brazil.