Between 1 July 1984 and 30 June 1986 all children treated for acute hematologic malignancy at our center were randomized to receive continuous (group A) or intermittent (3 days/week, group B) prophylaxis with trimethoprim-sulfamethoxazole (5-25 mg/kg/day/p.o.) against interstitial pneumonia with the aim of investigating if an intermittent regimen is as effective as and less toxic than a continuous regimen. The number of severe infections (group A, 17; group B, 21) and side-effects (group A, 30; group B, 34) was similar in the two groups, and compliance was also similar. We conclude therefore that neither regimen offers advantages over the other and the decision which to use should be based on cost (where regimen B has the advantage) and the children's and parents' preferences and compliance.