Histiocytosis-X cells were obtained at autopsy from the lungs and lymph nodes of a patient who died of the disseminated infantile form of this disease (Letterer-Siwe disease). The ability of these cells to synthesize and release prostaglandins was investigated in culture, by prelabeling the cell lipids with [14C] arachidonic acid and measuring the subsequent release of radioactive metabolites. The cells were seen to release primarily PGD2 and thromboxane. Correlative morphologic studies ensured the purity of the cell preparations, ruling out extraneous origin of the prostaglandins from sources other than the lesional histiocytes. Electron-microscopic study confirmed that the same cells that release prostaglandins and are capable of engulfing particles also bear the Langerhans' inclusions considered to be cell markers of histiocytosis-X. The prostaglandin production profile of alveolar macrophages from an infant who died as a result of congestive heart failure, but without histiocytosis, was studied for comparison. These cells produced PGE2 and thromboxane, but not PGD2. The theoretical implications of these findings are discussed.