Prior authors had hypothesized (but not clearly found) an increased apparent volume of distribution (Vd) for gentamicin in neonates undergoing extracorporeal membrane oxygenation (ECMO). We chose to study the question in our own clinical setting. To develop population pharmacokinetic models of the drug, we used the nonparametric expectation and maximization population modeling method and data from 11 neonates who received gentamicin on ECMO, including 6 infants who received gentamicin both on and off ECMO for severe respiratory failure. We found an increased Vd for gentamicin on ECMO and attributed much of the difference from prior investigations to our use of an explicitly determined laboratory assay error pattern for the measured serum concentrations rather than using constant weighting of the serum level data points. For six infants, while on ECMO their median Vd was 0.748 L/kg compared with a median Vd of 0.471 L/kg after ECMO was discontinued. The median clearance of gentamicin in the six infants while undergoing ECMO was 0.239 L/h compared with 0.350 L/h after ECMO was discontinued. The median half-time (T1/2) was 9.24 h while on ECMO compared with 3.87 h when off ECMO. We conclude that while undergoing ECMO, neonates have a higher volume of distribution for gentamicin, a lower clearance, and a much longer half-life.(ABSTRACT TRUNCATED AT 250 WORDS)