The systemic availability of oral zidovudine has been studied in 13 patients with the acquired immunodeficiency syndrome (AIDS) dosed either fasting or with breakfast. The mean peak plasma concentration and AUC of zidovudine were significantly 2.8- and 1.4-times higher in fasting patients than in those treated during meal. In both conditions the mean half-life was about 1.5 h and the period of plasma zidovudine concentrations greater than 1 mumol.l-1 was 2 h (NS). It is concluded that if zidovudine is taken on an empty stomach, high peak plasma concentrations and decreased variation in pharmacological parameters may be expected. Whether or not this will influence toxicity and efficacy remains to be shown.