The usefulness of transbronchial biopsy in diagnosing idiopathic pulmonary alveolar lipoproteinosis (PAL) is not emphasized in the literature. Therefore, we decided to reassess our approach to the morphologic diagnosis of this disorder in 14 patients diagnosed over the past 13 years in two major teaching hospitals in Toronto. The morphologic diagnosis of idiopathic PAL was established by means of open lung biopsy in 11 patients; the use of transbronchial biopsy was not considered in eight of them. Transbronchial biopsy was performed in six patients, and the diagnosis of idiopathic PAL was reliably established in five cases. We conclude that transbronchial biopsy may be a worthwhile preliminary procedure to open lung biopsy in patients with suspected idiopathic PAL, and it can reliably establish the diagnosis in such cases.