Anatomic and clinical observations of 28 cases, including 23 previously unpublished, of pulmonary lymphangiomyomatosis are recorded and discussed. This brings the total reported to 57. All patients were women in the reproductive age group with the major complaint of breathlessness. This was usually progressive, and death from pulmonary insufficiency resulted within 10 years. Functional changes were obstructive or restrictive, or both. Pneumothorax, chylous effusions and hemoptysis were frequent complications. Radiographically the lesions initially appear as fine, linear and nodular, predominantly basal densities, and progress to a pattern of bullous change, or honeycombing, involving all portions of the lungs not sparing the region of the costophrenic sinuses as is typical of eosinophilic granuloma. There may be associated pleural effusions. A progressively increasing lung volume is characteristic. The lesions consist of an irregular, nodular or laminar "irrational" proliferation of smooth muscle within all portions of the lung, with loss of parenchyma leading to honeycombing. Proliferated muscle can obstruct bronchioles (with air trapping and formation of bullae often complicated by pneumothorax), venules (with pulmonary hemorrhage and hemosiderosis accompanied clinically by hemoptysis) and lymphatics (with chylothorax or chyloperitoneum). Both thoracic and abdominal lymph nodes and the thoracic duct can also be involved in the myoproliferative process with formation of subsidiary minute channels and obstruction. Renal or perirenal angiomyolipomas can also occur, as exemplified by 2 patients in the present series. Identical pulmonary lesions occasionally occur in tuberous sclerosis. Especially since these patients usually have no neurologic disturbances and are almost women, the possibility of a relationship between tuberous sclerosis and lymphangiomyomatosis must be considered. One feature of note in pulmonary lesions of tuberous sclerosis is the presence of adenomatoid proliferations of epithelium. Such changes were also observed in 2 patients of the present series, and it is remarkable that both of these women had "retarded"children. At present the question of whether by lymphangiomyomatosis is a forme fruste of tuberous sclerosis must be considered as unresolved. It may yield to further investigation, possibility including chromosomal studies.