Total kidney volume, i.e. the combined volume of both kidneys (TKV), was determined by computerized tomography (CT scanning) during replacement therapy of terminal renal failure in 12 patients with adult polycystic kidney disease. Mean TKV was 2 090 (872-3437) cm3 in six of 11 patients with both kidneys in situ, treated for 44 (28-59) months, and 1 184 (308-2761) cm3 in five, treated for 97 (75-130) months. The difference is not statistically significant (0.1 greater than p greater than 0.05). Two patients, treated for 89 and 130 months, had a TKV of 308 and 408 cm3, respectively, i.e. almost equal to the normal (303 cm3 (SD 29)). Six patients, five with both kidneys in situ and one unilaterally nephrectomized, were examined with two CT scans at an interval of 31 (17-46) months and all showed a significant decrease in TKV, averaging 0.77 (0.49-1.09) % per month of treatment. It is concluded that polycystic kidneys undergo involution during replacement therapy, but the gradual shrinkage may be difficult to verify by single volumetric measurements in patients treated for varying periods due to the large interindividual variation in TKV in adult polycystic kidney disease.