The therapeutic activity of rimantadine and its relationship to the shedding of drug-resistant influenza A virus were assessed in two randomized, double-blind, placebo-controlled trials involving patients with laboratory-documented influenza A virus (H3N2 subtype) illness of 2 days' duration or less. In a family-based study, rimantadine treatment for 10 days (24 children and adults) was associated with significant decreases in the number of days to a 50% reduction in symptoms (mean difference, 2.5 days), days of fever (1.6 days), and days of restricted activity (1.5 days) compared with the results obtained with placebo-treated patients (32 children and adults). Drug-resistant virus was recovered from eight (33%) of the rimantadine recipients on day 5. No differences in patient demographics or illness severity at the time of enrollment in the study were apparent between those who shed resistant virus and those who did not. Illness resolution tended to be slower in those who shed resistant virus compared with that in those who did not. In a study of adults treated for 5 days (six treated with rimantadine, six treated with placebo), resistant virus was recovered in three rimantadine recipients by day 3 of treatment. The results indicate that drug-resistant influenza A virus (H3N2) can be recovered from rimantadine-treated children and adults as early as 2 days after starting treatment, but that rimantadine retains a net therapeutic benefit compared with that of placebo.