In order to optimize the results of kidney transplant, i.e. patient's acceptance and speedy recovery, the available organ resources must be combined with appropriate "preparation" of the patient's mental state. In order to plot the "mental deterioration curve" of patients on a hemodialysis program, the present study was made on 62 patients in a general hospital in Spain. The possible repercussions of treatment of kidney disease on intellectual (Weschler's Adult Intelligence Scale-WAIS test) and perceptive (Benton test) capacity were investigated. Samples were grouped according to whether they had been on dialysis for up to four years (less than 4) or more than four years (greater than 4). The WAIS test for the greater than 4 group indicated a lower I.Q. These results indicated that it was after four years on dialysis that greater negative effects began to be seen in perceptive-intellectual capacity. The Benton test indicated that time on hemodialysis adversely affected the patient's capacities. A sharp decline was seen particularly between two and four years in the less than 4 group, suggesting that patients' mental conditions vary with time on dialysis but that their resources and capacities are still fairly untroubled by the treatment in the first two years. In the light of these findings, the deterioration curve should be taken into account when planning kidney transplant, in order to take advantage of the initial period if possible.