In random order, a single dose of either tinidazole or metronidazole, calculated as 50mg/kg body weight, was given to 100 children with symptomatic giardiasis. Follow-up was done for 16 days and consisted of 4 clinical, parasitological and biochemical assessments at intervals of 4 days. The actual mean dose of the drugs received by the patients was 55.5 mg/kg of tinidazole and 52.0mg/kg of metronidazole. Success, i.e. parasitic clearance and symptomatic relief, was achieved in 80% of patients given tinidazole as opposed to 36% of patients given metronidazole (p less than 0.01). Moreover, control of diarrhoea and negative stool conversion for G. lamblia occurred earlier with tinidazole than with metronidazole (p less than 0.01). The incidence of gastrointestinal side-effects was 12% for tinidazole and 4% for metronidazole. Neither drug caused any abnormality of the blood count, blood urea, urine analysis or liver function tests.