Atazanavir is the most recently introduced protease inhibitor for the suppression of the anti-human immunodefficiency virus. A selective reversed-phase liquid chromatographic assay for this drug in human plasma has been developed and validated. Atazanavir was isolated from a 500 μL plasma sample using liquid-liquid extraction with dichloromethane. After evaporation and reconstitution of the extract the sample was analysed using liquid chromatography and ultraviolet detection at 280 nm. In the evaluated concentration range (44-4395 ng/mL atazanavir), intra-day precisions were ≤7% and inter-day precisions were ≤14%. Accuracies between 96 and 106% were found. The lower limit of quantification was 44 ng/mL with an intra-day precision of 7%, an inter-day precision of 14% and an accuracy of 87%. There was no interference from 32 tested potentially co-administrated drugs and metabolites. The usefulness of the assay was demonstrated for samples obtained from an HIV-infected patient treated with atazanavir.