Bronchiolitis obliterans with organizing pneumonia (BOOP) is an infrequently encountered clinical condition that can mimic a number of other pathologic lung processes. The presentation of this treatable condition in cancer patients has not been described in any large series. We conducted a retrospective study of patients with BOOP at Memorial Sloan-Kettering Cancer Center, NewYork, NY, U.S.A. from January 1992 to December 1999. The type and treatment of primary cancer, clinical and radiographic features of initial BOOP presentation, and outcome following therapy were recorded. Forty-three patients with an underlying diagnosis of cancer were found on lung biopsy to have BOOP as an isolated entity. BOOP was encountered in patients with a variety of clinical presentations, and many types of malignancies. The symptom patterns were non-specific, as were the physiological abnormalities. The only clear relationship between the underlying malignancyand the diagnosis of BOOP at presentation was in the chest radiographic findings. Patients with solid organ tumors were more likely to have nodular or mass like radiographic abnormalities (81%) than to have diffuse infiltrates (19%). We observed the opposite pattern in patients with hematologic malignancies (22% vs.67%). The vast majority of patients recovered from this condition. In conclusion, For cancer patients, BOOP represents a treatable cause of lung disease with protean manifestations. BOOP can mimic pulmonary malignancy and pulmonary infection. In cancer patients, the evaluation of new pulmonary symptoms accompanied by radiographic changes should include a consideration of this diagnosis.