Five cases are described in which the lungs were involved by ova of Schistosoma mansoni. Three of these individuals had evidences of chronic cor pulmonale at death. By the use of reconstructions made from serial sections, two types of angiomatoid lesions were found in the lungs. One type was found only in the cases with cor pulmonale. Dilated segments of the pulmonary arterial bed were replaced by connective tissue through which passed many, tiny vascular channels having a plexiform pattern. Part of the high pulmonary vascular resistance in these individuals was ascribed to constriction of the arterial bed in these areas. The development of these lesions was apparently unrelated to the direct deposition or movement of schistosome ova. The second type of angiomatoid lesions did not appear to influence pulmonary vascular resistance, being outside the pulmonary arterial bed. It was directly related to ova and the granulomatous reaction which surrounds them.