Two hundred and forty-five children infected with Trichuris trichiura, 143 of whom were also infected with Ascaris lumbricoides, were treated with albendazole as follows: single doses of either 600 mg or 800 mg, or daily doses of 400 mg for either 3 d or 5 d. Three stools were examined over a period of 10 d after treatment and again after nearly 40 d, using a quantitative microscopical technique. Albendazole appeared to act slowly against A. lumbricoides but within 10 d all dosages had cured about 92% of infections. In contrast, albendazole appeared initially to inhibit egg production by T. trichiura, which was then resumed. The single doses gave poor cure and egg reduction rates for T. trichiura of less than 30% each, and 400 mg of albendazole for 3 d was required to achieve a cure rate of 80%. The implications of these results are briefly discussed.