Forty-five patients with parasitologically confirmed symptomatic giardiasis were treated with tinidazole. A course of 150 mg twice daily for 7 days cured 14 of 19 patients (74%), and a single dose of 2,000 mg cured 24 of 26 (92%). After the single dose, mild side effects were common including maltaste, lassitude, and dizziness. Three probable cases of the so-called “postgiardiac syndrome” were seen. Either dosage of tinidazole initially cleared all stool samples of Giardia lamblia, and most clinical and parasitological failures were first detected a few weeks after the treatment. This emphasizes the significance of long follow-up periods.