A series of 18 patients with chronic Chagas' disease and human immunodeficiency virus infection were followed up for 2 to 66 months (median, 15.5 months). Artificial xenodiagnosis was positive for 81.3% and natural xenodiagnosis was positive for 81.8% of patients for whom they were performed; 81.8% of 11 patients had medium- or high-level parasitemia. Reactivation of Chagas' disease--direct microscopic examination of blood revealing parasites and, clinically, patients presenting with cardiac disease--occurred in three patients (16.7%). Specific antitrypanosomal treatment with benznidazole was effective in reducing the level of parasitemia and improving the clinical condition in three of the four patients treated.