The bioavailability of didanosine at 180 mg/m(2) once daily was compared to that at 90 mg/m(2) twice daily in 24 children with advanced human immunodeficiency virus infection. Children were studied at steady state using optimal sampling and prior pharmacokinetic parameter estimates. Relative bioavailability was 0. 95 +/- 0.49, supporting the potential clinical adequacy of once-daily dosing.