Twenty patients (18 M, 2 F; age 16-32 yrs) with symptomatic giardiasis were treated with tinidazole: ten each with a dose of 150 mg twice daily for 7 days, and a single dose of 2 g (50 mg/kg). Stool examination by formalin-ether concentration method was carried out before and one month after treatment. Cure was defined as absence of G lamblia in the stools. Fecal fat excretion (100 g load) and D-xylose absorption (5 g load) were measured before treatment and, if abnormal, repeated one month after treatment. Gut transit time was measured before and one month after treatment by giving radio-opaque beads orally and following their progress fluoroscopically to complete elimination. Parasitologically, all 20 patients were cured. Clinically, there was marked to complete relief of symptoms. Concomitantly, there was significant (p less than 0.01) reduction in fecal fat excretion, improvement in D-xylose absorption, and increase in gut transit time.