SUmmARY A working definition of pulmonary hypoplasia (PH) was established by retrospective assessment of lung growth both in recognised and hypothetical PH-associated conditions. Lung weight: body weight ratios (LW:BW) were calculated, and morphometry was determined by the radial alveolar count (RAC) (Emery and Mithal, 1960). Both parameters were reduced compared with those of normal controls in diaphragmatic hernia, anencephalus, anuric renal anomalies, chondrodystrophies, and osteogenesis inperfecta. Comparison of LW:BW ratio and RAC indicated that the RAC was the more reliable criterion of PH, LW:BW ratio of .0-012 (67% of mean normal ratio) and/or RAC of < 4.1 (75 % of mean normal count) are suggested as diagnostic criteria of PH. Evidence of PH was incidentally discovered in a number of clinically unsuspected cases and retrospectively clarified the clinical and radiological findings. Routine assessment of lung growth should be an essential part of the neonatal necropsy.