50 patients with giardiasis were treated with either metronidazole or tinidazole in random order according to a code. 25 patients received 2.0 g metronidazole on 2 consecutive days; 19 were cured (76%); 7 patients complained of mild side-effects. 25 patients received tinidazole 2.0 g as a single dose, and 22 were cured (88%); 5 patients complained of mild side-effects. The difference between the results of the two regimens was not statistically significant. A 1-week course of metronidazole (200 mg 3 times daily) was successful in 8/9 failures; 1 patient was finally cured with mepacrine.