Diurnal variation in plasma concentrations of nelfinavir and ritonavir has previously been described. Plasma concentrations of 29 pharmacokinetic profiles with indinavir, amprenavir and saquinavir were evaluated, and a statistically significant median difference of 57.6% was found between morning and evening, with the morning C-through being higher than the evening C-through.Consequently, a change in dosing intervals might result in more balanced plasma protease inhibitor concentrations, thereby decreasing toxicity and the risk of virological failure.