Cell surface markers are becoming increasingly important in the diagnosis of malignant lymphoid diseases. We present a case of pulmonary histoplasmosis with a pleural effusion. The differential diagnosis included non-Hodgkin lymphoma because the pleural fluid cells were cytologically identical to convoluted lymphoblasts; the cells also formed rosettes with sheep erythrocytes at 37 degrees C, suggesting that they were malignant thymus-derived lymphoblasts. Since cultures of pleural fluid were negative for bacteria and fungi, the correct diagnosis of histoplasmosis was made only after conventional histology identified Histoplasma capsulatum organisms in pleural nodules. Thus, until we have a better understanding of the significance of cell surface markers, we should continue to rely on conventional histology for the diagnosis of lymphomas.