Severe segmental renal atrophy with loss of parenchymal elements in small kidneys is commonly known as segmental hypoplasia. The scars are seen as cortical depressions overlying shrunken medullary pyramids and their dilated calyces, and are characterized histologically by colloid-filled tubular microcysts and a paucity or absence of glomeruli. This lesion has been identified in 17 patients, 11 female and 6 male, between 6 and 23 years of age. Eleven patients had hypertension, which developed in six while they were under observation. Thirteen had histories of urinary tract infection, and 16 had evidence of vesicoureteric reflux. Seven patients had impaired renal function (GFR less than 40 ml/minute/1.73 m2). Abnormal metanephric differentiation (dysplasia) in two specimens, one in association with posterior urethral valves, suggested an occasional intrauterine origin of the abnormality. Twelve patients had radiographic evidence of decreasing renal size over two to five years of observation, even after surgical correction of reflux, in four of them unaccompanied by infection. We conclude that segmental "hypoplasia" is an acquired lesion, although it sometimes has intrauterine origins, and that it is commonly associated with vesicoureteric reflux, even in the absence of demonstrable infection.