We report five patients diagnosed of pulmonary hypoplasia (PH) in adulthood (age > 18 years, range 45 to 67 years). PH was left-sided in four cases. The malformations found were 2 cases of renal agenesis, 2 diaphragmatic hernias and 1 hiatal hernia. Lung function data were FEV1 0.88 +/- 0.32 L; FVC 1.30 +/- 0.52 L; FEV 1/FVC 68.5 +/- 5%. Arterial gases (with FiO2 0.21) were PaO2 63.6 +/- 24 and PaCO2 47.1 +/- 11.2 mmHg. Three cases were referred for monitoring of respiratory insufficiency (2 requiring home oxygen therapy) and two for radiographic study of chest disease. The diagnoses were made by computerized tomography (CT) of the chest. Congenital diseases due to underdeveloped lungs in the pseudoglandular phase are rarely diagnosed in adults (48 cases, including ours, have been found in the literature), and PH is rarely diagnosed, possibly because the anomalies observed are attributed to old infections. Clinical presentation is highly variable, depending in large measure on a history of smoking and repeated respiratory infections. Chest CT is at present the diagnostic tool of choice.