The efficacy of oral pivmecillinam was compared with nalidixic acid in the treatment of acute shigellosis in children 1-8 years of age. In a double-blind trial we studied 80 comparable children with bloody diarrhoea of less than 3 days' duration. Shigella spp. was isolated in 71 children. Patients were randomly assigned to receive either pivmecillinam, 50 mg/kg.day, or nalidixic acid, 60 mg/kg.day, both given orally for 5 days. The stool frequency decreased progressively in both treatment groups. Nalidixic acid failed to eradicate Shigella species in 10 patients, compared with three in the pivmecillinam group (p = 0.04). Similarly, clinical failure was observed in 11 of 37 patients receiving nalidixic acid and in 2 of 26 patients infected with nalidixic acid-susceptible strains as against none in the group receiving pivmecillinam. The results suggest that pivmecillinam given orally was, in fact, more effective than nalidixic acid in the treatment of acute shigellosis in children, particularly when the resistant strains are taken into account.