Pharmacokinetic studies were conducted with human immunodeficiency virus-infected patients receiving efavirenz, nelfinavir, or both agents at weeks 4 and 32. Reductions of 25% and 45% were observed in the mean nelfinavir area under the concentration-time curve and minimum concentration of the drug in serum, and there was a 31% more rapid half-life for patients receiving both drugs compared to patients receiving nelfinavir alone. There were no significant differences in efavirenz pharmacokinetics.