A microhematocrit concentration method (MH) for immediate diagnosis of Chagas' disease during the acute stage or in congenital cases was standardized. Parasitemia as low as 1,000 parasites per ml was detected, after centrifugation of six 50-pl capillary tubes, by 10-min microscopic observation of each buffy coat spread between slide and cover glass. Operator's time was reduced by at least one-third when compared with a fresh blood observation (FB). In 12 of the 15 patients studied, diagnosis was performed in 4.9 ± 3.08 min with MH, whereas 27.0 ± 12.1 min were necessary when FB was used. In the three remaining patients whose FB results were negative, MH became positive after 13, 16, and 40 min. In our experience, FB proved to be more sensitive than previously reported. Suckling mouse inoculation also proved to be sensitive but, as in xenodiagnosis and in hemoculture, the delay in getting the final result was a limiting factor.