Eight patients with hematologic malignant disease presented with associated alveolar proteinosis. All but one of these patients also had evidence of opportunistic infections. No explanation for this association could be elicited, but all patients appeared immunosuppressed at the time of their death. Autopsy revealed proteinaceous material or micro-organisms in the alveoli, but the inflammatory response was minimal. Forty cases of opportunistic infections and 22 cases of hematologic malignant disease were found among the patients with alveolar proteinosis reported in the literature. Lungs in these cases also lacked a significant number of inflammatory cells capable of phagocytosis. On the basis of these observations, a hypothesis is formulated to explain alveolar proteinosis as being a result of poor alveolar clearance in immunosuppressed patients.