To establish if antibodies to alpha smooth muscle actin (ASMA) are better than desmin as a tumour marker in pulmonary lymphangioleiomyomatosis both antisera were applied to five cases. ASMA strongly stained the muscle in all cases but desmin was negative. Five cases of cryptogenic fibrosing alveolitis showed that mature smooth muscle embedded in fibrous tissue surrounding cysts was positively stained with ASMA but the interstitial fibrous tissue was negative. ASMA is a consistent and better marker than desmin for the detection of immature smooth muscle in pulmonary lymphangioleiomyomatosis.