Eleven patients had radiologically non-functioning and unilaterally obstructed kidneys with normal kidneys on the other side. The kidneys were divided into three groups according to the urinary pH of the obstructed kidney at operation: Group 1 (pH 6.0 or less); Group 2 (pH 6.1-7.1); Group 3 (pH greater than 7.1). The obstruction was corrected in all patients despite the pre-operative radiological findings or the morphological appearance at operation. Radiological recovery was assessed 2 months post-operatively and was found to be good in all patients in Group 1. In Group 2, recovery was good in one patient, moderate in two and absent in one. In Group 3 there was no evidence of radiological recovery in either patient.