Urinary pyridinoline (Pyr) and deoxypyridinoline (Dpyr) are collagen crosslinks found in bone and cartilage and are sensitive bone resorption markers. In this study, values of urinary Pyr and Dpyr during 24-h periods in 6 normal men, 10 normal women, and 10 osteoporotic patients were measured. In these three groups, urinary Pyr and Dpyr showed significant circadian variations only in normal men. Neither urinary Pyr nor Dpyr showed significant circadian variations in normal women or osteoporotic subjects, although they had the same trend as the diurnal changes of urinary crosslinks in normal men. Comparing the values of urinary crosslinks in the daytime (11:00–20:00 h) and at night (20:00–11:00 h), the latter were significantly higher than the former in both normal and osteoporotic subjects. Furthermore, the nighttime and daytime difference of urinary Pyr in osteoporotic subjects was significantly larger than those in normal subjects. Urinary Pyr and Dpyr were 22.6% and 10.7% higher, respectively, at night compared to during the day in normal women, while in osteoporotic patients, the values were 51.2% and 25.5% higher at night than during the day, respectively. In conclusion, the acceleration of bone resorption at night may be one of the etiologies of involutional osteoporosis.