Fifty-nine women with trichomonal vaginitis were randomly allocated to receive treatment with a single oral dose of either 0.5, 1.0, or 1.5 g of ornidazole. One week after treatment, a parasitologic cure was observed in 100% of patients treated with 1.5 g, in 95% of patients treated with 1.0 g, and in 65% of patients given 0.5 g. At the one-month follow-up visit, the cure rate remained at 100% for the 1.5-g dose group but dropped to 85 and 45% in the 1.0- and 0.5-g dose groups, respectively. The disappearance of symptomatic complaints was also dose related: the clinical cure was 100, 85, and 40% at the first follow-up visit and 89, 80, and 30% at the second follow-up visit. Adverse effects of mild or moderate severity were reported by 13 patients. These were encountered mostly in the 1.5- and the 1.0-g dose groups. The most frequent adverse effects were dizziness and gastrointestinal distress. Laboratory safety test did not reveal any significant toxicity. This study confirms that single-dose treatment of trichomoniasis with an oral dose of 1.5 or 1.0 g of ornidazole is effective and well tolerated.