Our aim was to describe the incidence, clinical course, and risk factors for idiopathic pneumonia syndrome (IPS) after high-dose chemotherapy with cyclophosphamide, carmustine, and thiotepa followed by autologous stem cell transplantation for high-risk breast cancer. Charts for patients who underwent high-dose chemotherapy for high-risk breast cancer at a single center from 1992 to 2000 were retrospectively reviewed, and potential risk factors for development of IPS were sought with the log-rank test. Of 164 patients reviewed, 20 developed IPS at a median onset of 87 days after the transplant (range, 2-257 days). The actuarial incidence of IPS in the first 100 days after the transplant was 8%, and 95% of patients developed symptoms within the first 6 months after transplant. Patient age, smoking status, breast cancer stage at diagnosis, and pretransplant lung function did not predict development of IPS. Three patients died of progressive pulmonary failure and the IPS resolved in the other 17. We concluded that IPS is an important cause of morbidity and mortality in patients with high-risk breast cancer undergoing high-dose chemotherapy. Given the absence of predictive factors, any pulmonary symptoms appearing in the first year after the transplant should be evaluated carefully.