While bronchiolitis obliterans organizing pneumonia (BOOP) has been associated with the use of sirolimus (SIR), the incidence in a consecutive group of patients given SIR to replace a calcineurin-inhibitor (CI) is unknown. Twenty-nine consecutive cardiac transplant recipients were switched from a CI to SIR to ameliorate CI-associated nephropathy or coronary graft atherosclerosis. Seven patients (24%) developed BOOP. The clinical characteristics and biopsy results of these patients are presented. The clinical course and response to withdrawal of SIR in all and steroids in four of seven patients suggested the diagnosis of BOOP. Chest X-rays and CT scans showed typical findings of BOOP in all seven patients. Infection was excluded in all patients. Biopsy results were characteristic of BOOP in six of seven patients. Six patients recovered and one died. BOOP is a common and potentially serious adverse event in cardiac transplant patients switched from a CI to SIR, especially when SIR is started late post-transplantation.