Timeliness and retention in a 6-month follow-up study were analyzed by subjects' baseline characteristics in a seroincidence study of 263 injection drug users at high risk of human immunodeficiency virus infection. Subjects were recruited from September 1997 to June 1998 in community settings in Philadelphia, Pennsylvania. Of these subjects, 93% were completers: 11% before the targeted date, 38% ... [Show full abstract] at the targeted date, 32% within 1 month of delay, and 12% beyond 1 month. Late completers were more likely than other completers to be younger and to live farther away from the study center, less likely to have stayed in a shelter or a welfare residence during the past year, more likely to have a lower income, and more likely to have shared rinse water, cotton, or cooker. By contrast, loss to follow-up was not associated with these variables. Subjects lost to follow-up were more likely than those retained to have a high school diploma and to have moved during the past year; their source of needles was less likely to be a needle exchange program and more likely to be a shooting gallery. None of the drug-related behaviors that increase the risk of human immunodeficiency virus infection was associated with timeliness or retention, suggesting that the study might be minimally biased. bias (epidemiology); follow-up studies; HIV; substance abuse, intravenous Abbreviation: HIV, human immunodeficiency virus.