Ornidazole or tinidazole as a single dose of 1.5 g was given to 100 patients with symptomatic giardiasis. Parasites disappeared from the feces of all patients, usually within 2 or 3 days, and symptoms subsided in 98 cases, usually within 5 days. During an 8-wk follow-up, parasites reappeared in 10 patients, 5 after each drug, between 1 and 7 wk after treatment. The relapses were usually symptomatic, and the probability that they were reinfections was negligible. Ornidazole appeared in serum more rapidly than tinidazole, and the mean elimination half-lives were 10.9 and 11.1 h, respectively. Subjective side effects were equally common after both nitroimidazoles; dizziness was associated with rapid initial absorption. We found no correlation between drug absorption or elimination and therapeutic efficacy. Both drugs are recommendable as a single oral dose of 1.5 g in giardiasis.