The follow-up of renal function in kidney transplant patients requires sensitive and specific parameters which allow the detection of clinically significant changes. In this prospective study, we have evaluated the accuracy of the urinary protein/creatinine ratio (UP/UCreat), determined in morning urine specimens, in assessing 24-hour proteinuria (P24). Fivehundred and twenty paired samples were provided by 133 kidney transplant patients. The correlation coefficient of the linear regression was 0.93 in both in the first set of paired samples (133 samples) and in all paired samples (520 samples). In complete urine collections, the UP/UCreat predicted the level of proteinuria with both a good specificity (95 to 99%) and sensitivity (97 to 99%) at different levels of 24-hour protein excretion. The intraindividual coefficients of variation of P24 and UP/UCreat were evaluated in 82 patients (442 samples) and were 23% and 29% respectively. These results confirm that the UP/UCreat ratio in morning samples is a reliable estimate of the 24-hour proteinuria in kidney transplant patients. Additionally, its variation appears to accurately reflect changes in the rate of protein excretion.