The aim of this study was to compare the pharmacokinetics of idarubicin (IDA) and its active metabolite idarubicinol (IDOL) after chronic oral and continuous intravenous (i.v.) IDA administration in order to establish the oral doses needed to reach the i.v. equiactive plasma drug exposure. The pharmacokinetic profile of IDA and IDOL was investigated in 23 patients receiving 12 mg/m2 IDA by 120-h i.v. infusion (2.4 mg/m2/day) combined with cyclophosphamide, etoposide and prednisone in comparison to 28 patients receiving oral IDA doses ranging from 2 to 10 mg/day for 21 days in a phase I study. We found that IDA AUC24h/dose/m2 was 4.7-fold greater during i.v. than oral administration, whereas IDOL AUC24h/dose/m2 was only about 2-fold higher after i.v. administration. The metabolic ratio between IDOL AUC24h and IDA AUC24h in plasma was about 3-fold higher after oral administration. Based on these results we were able to estimate that equiactive plasma drug exposure was reached with an approximately 2.5-fold greater oral dose/m2 of IDA than the corresponding i.v. dose.