To determine the ability of monocyte phagosomal acidification in chronic paracoccidioidomycosis, 13 patients were recruited at different times during follow-up and compared with 18 normal controls. Eight patients were studied at diagnosis, six of them also during treatment and five additional patients after ending treatment. Phagosomal acidification of monocytes, triggered by challenge with opsonized zymosan, was evaluated with acridine orange and expressed as percentage of orange-stained intracellular particles, as mean +/- SE. In comparison with controls, acidification was severely impaired before treatment (33 +/- 11% vs. 67 +/- 6%) and reached values similar to controls during treatment (73 +/- 6%, n = 6). In addition, phagosomal acidification of the patients studied after treatment (63 +/- 4%) had no difference when compared with controls. This study demonstrates that phagosomal acidification is perturbed among chronic paracoccidioidomycosis patients and reverses with antifungal treatment.