Renal volume was determined by repeated CT scanning in 26 patients with adult polycystic kidney disease after institution of replacement therapy for terminal renal failure. Thirteen patients, treated for an average period of 6.7 years (SD 3.4, range 1.1-13.7), had a functioning necrokidney graft at the close of the study and had been grafted for 80.8% (SD 18) of the total treatment time. They showed a reduction of renal volume averaging 1.07% (SD 0.89) per month. Thirteen patients, treated for an average period of 5.7 years (SD 3.3, range 1.2-12.2), were on dialysis at the close of the study. Four had been on dialysis only, 9 had carried one or more (rejected) necrokidney grafts for 22.8% (SD 25) of the total treatment time. Three of them showed a significant increase in renal volume, whereas ten showed a decrease. The average change of renal volume in the entire group was -0.28% (SD 1.36) per month, significantly smaller than in the transplanted patients (p less than 0.01). It is concluded that polycystic kidneys shrink after successful grafting, whereas volume changes vary on dialysis treatment.