In an attempt to improve the diagnostic value of urine analysis in patients with homozygous cystinuria, we studied the diurnal variation in urine composition. A simplified estimate of the ion-activity product of cystine was used to increase the probability of identifying patients with a particular risk of stone formation. Eight 6-h urine samples were collected during two 24-h periods. The highest urinary excretion of cystine was recorded between 1200 and 1800 hours and the lowest between 0000 and 0600 hours, whereas the urinary cystine concentration was highest between 0000 and 0006 hours and lowest between 1200 and 1800 hours. The approximate ion-activity product of cystine had a maximal level between 0000 and 0600 hours but a minimal level between 0600 and 1200 hours. The differences between different periods were numerically more pronounced in terms of the ion-activity product of cystine than in terms of concentration. The peak concentrations of cystine in 6-h samples were about 90% higher than the corresponding concentrations in 24-h urine samples. It is concluded that the analysis of cystine in 6-h urine samples reveals transient episodes of cystine supersaturation that otherwise will remain undetected. Further studies are, however, needed to establish its usefulness in clinical practice.