Granulomatous lesions similar to those of pulmonary histiocytosis in rats developed spontaneously in the lungs of captive tree shrews. Incidence peaked in 3-year-old tree shrews. Sex dependency was not observed, and development of the granulomas was unrelated to experimental procedures because the lesions were observed in animals from the breeding stock as well. The granulomas consisted of amorphous material, foam cells, and a few foreign body-type multinuclear giant cells; they also contained acicular clefts, often with some fibrous material. Alveolar septa within and adjacent to the granulomas were thickened in most instances, but did not contain inflammatory cells in appreciable numbers or amyloid. Only traces of cholesterol and calcium were detected in the amorphous material; neutral fat was stored in the foam cells and the amorphous masses. Fibers without birefringency were documented by polarization and scanning electron microscopy in the vicinity of granulomas, which in energy-dispersive X-ray microanalysis consisted mostly of calcium, but lacked silicon.