We report 2 cases of maculopapular eruption and fever in patients infected with human immunodeficiency virus (HIV) on the 2nd day of first administration of ritonavir, a protease inhibitor. In the 1st patient, clinical improvement occurred despite continuation of therapy, and in the 2nd the treatment was stopped with remission and rechallenge resulting in recurrence. Ritonavir should be added to the list of drugs that can induce adverse cutaneous reactions in HIV patients.