Lysinuric protein intolerance is an autosomal recessive disease caused by defective transport of cationic amino acids. Of the 38 lysinuric protein intolerance patients diagnosed in Finland since 1965, four pediatric patients have died. We describe the clinical courses and autopsy findings for these patients. All patients developed acute respiratory insufficiency. In addition to pulmonary hemorrhages, three of the patients had pulmonary alveolar proteinosis and one had cholesterol granulomas. Three patients had a clinically obvious renal insufficiency, but all four showed histologic signs of immune complex-mediated glomerulonephritis. The patients also developed hepatic insufficiency with fatty degeneration or cirrhosis. All patients showed anemia, thrombocytopenia, and a severe bleeding tendency. The bone marrow of three patients was hypercellular, but the amount of megakaryocytes was decreased in two cases. Amyloid was present in the lymph nodes and the spleen. Bone specimens showed osteoporosis. We conclude that pediatric patients with lysinuric protein intolerance are predisposed to develop pulmonary alveolar proteinosis and glomerulonephritis. They are also at risk of protein malnutrition in the active growth phase, probably due to higher requirements for total nitrogen and amino acids.