Interstitial pneumonitis (IP) occurred in 20 of 143 (14%) patients who received cytoreductive therapy followed by autologous bone marrow transplantation (BMT) as treatment for malignancy. IP occurred at a median onset time of 41 days (5 to 624 days). All but three of the episodes were fatal. Of the thirteen cases in which tissue was examined, half were idiopathic; the remainder were due to various infectious agents. The actuarial incidences of idiopathic (7%) and CMV IP (2%) in these marrow autograft recipients were lower than the incidences of idiopathic (19%) and CMV IP (17%) in comparably treated recipients of allogeneic BMT (P less than or equal to 0.001 for both comparisons).