One hundred children suffering from symptomatic giardiasis were treated with either tinidazole or metronidazole in random order. Both the drugs were given as a single oral dose calculated on the basis of 50 mg/kg body weight. Parasitological and clinical cure was obtained in 40 (80%) of 50 patients given tinidazole and in 18(36%) of 50 patients given metronidazole. This difference in cure rates was significant (p less than 0.01). Furthermore, control of diarrhoea and negative stool conversion for G. lamblia were achieved earlier with tinidazole than with metronidazole, the differences being significant (p less than 0.01) from the 8th post-treatment day. Gastro-intestinal side-effects of mild degree occurred in 6 patients on tinidazole and in 2 patients on metronidazole; they comprised nausea, vomiting, and bitter taste. Neither drug caused any abnormal deviation in blood counts or in biochemical tests of liver and kidney function.