Plasma prednisolone concentrations were measured in 26 renal transplant (RT) patients and five control subjects. A linear relationship was found between prednisolone dosage in mg/kg and area under the plasma concentration time curve in seven stable RT patients (r = 0.98) and in all subjects (n = 19) with estimated prednisolone clearance rates (CLp) between 0.075 and 0.11 L/kg/hr (r = 0.96). The greatest deviation from this occurred in five patients having reduced CLp with prolonged prednisolone half life (t1/2p) and in one having increased CLp while taking phenytoin. Three of the five with reduced clearance had steroid induced diabetes and one a Cushingoid appearance. There was some correlation between peak plasma prednisolone and dosage (r = 0.77) in all subjects, but none between creatinine clearance and t1/2p or CLp.