Hydroxyurea is used in the treatment of various forms of cancer, sickle-cell anaemia and HIV infection. Oral absorption of the drug is virtually complete, the volume of distribution is equivalent to total body water and elimination is through both renal and nonrenal mechanisms. Nonrenal elimination of hydroxyurea is characterised by Michaelis-Menten kinetics. Further studies are necessary to clarify several aspects of the pharmacokinetics and pharmacodynamics of hydroxyurea: the effect of age and disease state, concentration-effect relationship, the role of therapeutic drug monitoring, and the mechanisms of renal and nonrenal elimination. The recent development of improved assays for hydroxyurea should have benefits for future pharmacokinetic studies.