Albendazole, a broad spectrum anthelmintic recently shown to be active in vitro against Giardia duodenalis, was given at 4 different dosages and compared with metronidazole in the treatment of children in Bangladesh infected with Giardia. Three stools were collected over 10 d after treatment and examined microscopically. Albendazole was found to be effective: single doses of either 600 mg (n = 103) or 800 mg (n = 114) successfully treated 62% and 75% of infections, respectively; 400 mg given either once a day for 3 d (n = 116) or for 5 d (n = 115) successfully treated 81% and 95% of all infections, respectively. Albendazole given daily at 400 mg for 5 d was as effective as metronidazole, which cured 97% of infections (n = 230). Albendazole may thus be an alternative treatment for infections with Giardia, while the moderate efficacy of single doses may provide a benefit in addition to its effects on several species of intestinal helminths.