This study was designed to compare the treatment efficacy of single dose of ornidazole with 5 d treatments of ornidazole and metronidazole in children with giardiasis. 175 children, between 2 and 15 y old, whose stool samples were found to be positive for Giardia lamblia cysts and/or trophozoites by either saline-Lugol, formalin-ethyl acetate or trichrome staining, were enrolled in the study. Of these children, 105 were treated with a single dose of ornidazole: 35 with 30 mg/kg, 35 with 25 mg/kg and 35 with 20 mg/kg; 35 were treated with 25 mg/kg per day of ornidazole for 5 d in 2 doses and 35 children were treated with 20 mg/kg per day metronidazole for 7 d in 3 doses. All cases were examined on the 7th, 10th and 14th days after treatment by the same methods; clinical symptoms were also evaluated. Giardia lamblia was eradicated in 34 of 35 (97%), 34 of 35 (97%) and 33 of 35 (94%) patients treated with 30, 25 and 20 mg/kg single doses of ornidazole, respectively. Eradication was achieved in all 35 patients treated with 25 mg/kg per day ornidazole for 5 d and in 31 of 35 (89%) patients treated with metronidazole. There was no statistically significant difference among doses of ornidazole (p > 0.05); however, all ornidazole treatment regimens were significantly more effective than metronidazole treatment (p < 0.05). No important side-effects were detected in any patients and clinical symptoms disappeared in all. Single-dose ornidazole treatment could be considered as a proper and effective alternative method for the treatment of giardiasis in children.