The therapeutic effects of quinacrine (Atabrine) and metronidazole (Flagyl) were compared in a 3-year prospective study of 160 infants and children (86 boys and 74 girls ranging in age from 4.5 months to 13 years) with giardiasis. The most common symptom was recurrent abdominal pain. In each study group stool examinations were done 5 days, 1 month, and 6 months after treatment. There were no treatment failures with metronidazole, whereas four of those treated with quinacrine had positive stools 5 days after treatment, indicating possible failure. There were no recurrences at 1 month; after 6 months, however, Giardia infection was found in 13% of both treatment groups. These recurrences were seen mainly in children from families with other infected members. Considering the low failure rate, the minimal side effects, and the relatively more tolerable flavor, metronidazole seems to be preferrable in the treatment of giardiasis. A dosage of 15-25 mg/kg a day for 5 days is recommended.