Cyclosporin A (CsA) absorption is low and variable after liver transplantation, and during the intravenous-oral transition period CsA concentrations may not be maintained within the therapeutic range. Trough whole blood CsA concentrations were measured by high-performance liquid chromatography before and after the transition period in 27 liver transplant patients. Mean (SD) CsA concentrations decreased from 291 (92) to 198 (96) ng/ml (p < 0.001). When analyzed on individual charts, a decrease was observed in 15 of 27 patients and it was associated with a duration of 2 days or less (p < 0.01) and a total bilirubin value below 3.5 mg/dl (p < 0.05). Our results show that, despite therapeutic drug monitoring, CsA blood concentrations may decrease during the intravenous-oral transition period.