Pulmonary Alveolar Proteinosis (PAP) is a rare and diffuse lung disease characterized by the abnormal deposition of PAS positive, lipoproteinaceous material in the alveolar spaces. It has been related, mainly, to alterations in the immune state and to secondary infections. We studied four cases of PAP diagnosed by light microscopy. In two cases we were able to demonstrate disseminated Histoplasmosis related to immunodeficiency states (AIDS and malnutrition), one case with Pneumocystis carinii infection and AIDS, and one case with no related pathology. Granular and electron dense material, concentric myelin figures, and variable-sized osmiophilic bodies were observed by electron microscopy. We found yeast-like structures, trophozoites and cysts in the alveolar spaces, in the Histoplasmosis and Pneumocystic carinii infection cases, respectively. In one of our cases, the circulating neutrophils showed crystalloid inclusions in the nucleus. PAP should be considered in the differential diagnoses of patients with pulmonary infiltrates.