The relation between a number of potential risk factors and change in body mass index per semester was examined in a community-based cohort of 1,809 homosexsual and bisexual men seropositive for human immunodeficiency virus type 1 (HIV-1). The men were followed semiannually for up to 6.5 years between 1984 and 1990. A total of 9,735 person-semesters of observations were available for analysis. A Markov-type autoregressive model, adjusting for previous body mass index, was used to predict the change in body mass index over each person-semester. Overall, the cohort was gaining weight An asymptomatic participant 1.8m in height whose CD4+cell count was >750/μl gained a mean of 0.5 kg each person-semester. In bivariate autoregressive models, diarrhea, fever, oral thrush, acquired immunodeficiency syndrome (AIDS), and CD4+lymphocyte counts of p = 0.057), althougth AIDS (p = 0.009), fever (p = 0.006), thrush (p = 0.002), and a CD4+lymphocyte count of p < 0.001)all remained independently associated with a decrease in body mass index. Lymphadenopathy and a CD4+ lymphocyte count of 100–199 cells/μl were also significant covariates in the final model, but neither of the beta coefficients exceeded that of the intercept, indicating that they were not independently associated with a decrease in body mass index. These findings suggest that the importance of diarrhea as a cause of HIV-related weight loss may have been over -estimated in previous clinic-based studies. AIDS and nonspecific markers of progression (fever, thrush, and a CD4+ count of <100 cells/μl) were the best predictors of weight loss during a semester.