In pediatric liver transplant recipients, oral cyclosporine (CsA) therapy may be complicated by impaired or delayed absorption during the initial weeks posttransplant. Neoral (NL) is a microemulsion preconcentrate formulation of CsA expected to increase the rate and extent of absorption of CsA and have less pharmacokinetic variability. The absolute bioavailability (F) of CsA from NL was compared with that of the currently marketed Sandimmune (SM) formulation in a double-blind, crossover study conducted in 9 pediatric liver transplant recipients (age 6 months to 11 years) between 8 and 20 days posttransplant. After determination of CsA pharmacokinetics for a steady-state intravenous dose, patients were randomized to receive a single oral dose of NL or SM in period I and the alternative formulation in period II. Clearance (Clt) and volume of distribution (Vss) values (mean +/- s.d.) calculated from the i.v. dose were similar to that previously reported for pediatric patients (Clt = 12.0 +/- 1.3 ml/min/kg; Vss = 2.2 +/- 0.2 L/kg). Mean F (+/- SD) for NL was significantly higher than SM (NL = 37.6 +/- 14.6%; SM = 24.7 +/- 8.0%; P = 0.05). Although not reaching statistical significance, the observed maximum blood concentration (Cmax) was higher, and the time to Cmax (Tmax) was shorter for NL in 8 or 9 patients. There were no significant correlations between age and any pharmacokinetic parameter for the group as a whole--however, there were statistically significant correlations between age and F for NL (r = 0.87; P = 0.02), and for age and Vss (r = 0.91; P = 0.01) for the 6 patients aged 2 years or less. In this pediatric liver transplant population, Neoral demonstrated improved absorption (% increase in F) compared to Sandimmune. In liver transplant recipients aged 2 years or less, absorption of Neoral may be a function of age and/or bowel length.