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 homosexual 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 1.8 m in height whose CD4+ cell count was > 750/microliters 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 < 100 and 100-199 cells/microliters were all associated with a significant decrease in body mass index. A significant inverse association was also found between change in body mass index and lymphadenopathy and herpes zoster, but when the intercept coefficient was added, no overall decrease in body mass index was seen in these models. In a final multivariate model, diarrhea was less strongly associated with a change in body mass index (p = 0.057), although AIDS (p = 0.009), fever (p = 0.006), thrush (p = 0.002), and a CD4+ lymphocyte count of < 100 cells/microliters (p < 0.001) all remained independently associated with a decrease in body mass index. Lymphadenopathy and a CD4+ lymphocyte count of 100-199 cells/microliters 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/microliters) were the best predictors of weight loss during a semester.