Giardiasis is a cosmopolitan parasitosis. Diarrhea, abdominal colic, and flatulence are the main clinical symptoms, however, malabsorption, and impairment of growth of children may occur. The 5-nitroimidazoles are the drugs of choice in the treatment of giardiasis. Methods: The efficacy and tolerability of secnidazole and tinidazole were evaluated in a randomized, open-label, clinical trial performed with 267 Giardia lamblia-positive children. Secnidazole, in a new gel formulation, and tinidazole suspension were prescribed as single oral doses of 30mg/kg and 50mg/kg, respectively. Clinical and parasitological follow-up was carried out before, and at 7, 14, and 21 days after treatment. Results: Clinical cure was observed in 77.3% and 75.7% of the patients in the secnidazole and tinidazole groups, respectively. Parasitological cure was obtained in the 91.3% and 89.6% in the secnidazole and tinidazole groups, respectively. A metallic taste after drug ingestion was more commonly reported in the tinidazole group than in the secnidazole group (p<0.05). Conclusions: The authors conclude that both secnidazole gel and tinidazole administered as a single oral dose are effective treatments for children with giardiasis since both high cure rates and good tolerability were observed.