Residual renal function (RRF) is a key element in the management of chronic peritoneal dialysis (PD) patients, and 24-hour creatinine clearance (24-h Ccr) and arithmetic mean of creatinine and urea nitrogen clearances [24-h (Ccr+Curea)/2] are still standard clinical techniques for the assessment of glomerular filtration rate (GFR) to represent RRF. However, it is sometimes difficult to monitor urine collection for 24 hours, especially in outpatients, and it requires serum sampling. Therefore, we devised a new and simple method to measure RRF in prevalent PD patients. Levels of urinary cystatin C (Cys-C) and creatinine in spot urine samples [24-h (Ccr+Curea)/2] were measured in 15 stable Japanese PD patients. Although no statistical correlation was seen between Cys-C and 24-h (Ccr+Curea)/2 values, a strong correlation was found between the spot urine Cys-C-to-creatinine ratio [U (Cys-C/Cr)] and 24-h (Ccr+Curea)/2. By simple linear regression analysis, the following regression equation was derived: y = 17.0 - 6.1x, where x = log 10,000(*)U (Cys-C/Cr) ratio and y = 24-h (Ccr+Curea)/2. Measurement of U (Cys-C/Cr) ratio does not require serum sampling or 24-hour urine collection but requires only a spot urine sample. We suggest that the U (Cys-C/Cr) ratio is a simple and easy screening method to estimate GFR in PD patients.