The present study investigated whether overnight urine collection can replace 24-hour urine collection to measure urinary calcium (Ca) excretion in healthy individuals. One hundred healthy men (age 25-74 years) on their usual diet participated in the present study as part of an ongoing epidemiological population-based survey. Two separate bags (daytime and overnight bags) were given together with instructions to collect a complete 24-hour urine sample divided in to the daytime and the overnight specimen. Urinary Ca was analyzed by atomic absorption spectrophotometry and creatinine by the picric acid colorimetric method. Overnight urinary Ca was correlated to daytime (r = 0.618, p < 0.0001) and 24-hour urinary Ca (r = 0.891, p < 0.0001). In the 17 men found to have hypercalciuria (24-hour urinary Ca > or = 7.5 mmol), the overnight urinary Ca averaged 4.44 +/- 0.29 mmol/12 h (mean +/- SEM, range 2.35-6.38 mmol/12 h), indicating that an overnight urinary Ca of < 2.35 mmol/12 h (< 60% of the overnight urinary Ca distribution) ruled out the possibility of finding hypercalciuria in 24-hour urine. Fourteen of the seventeen hypercalciuric men had overnight urinary Ca in the upper 20% of the distribution (> or = 3.25 mmol/12 h). Hypercalciuria was found in 14 of the 19 men (73.7%) with an overnight urinary Ca of > or = 3.25 mmol/12 h, but only in 3 of the 81 men (3.7%) with an overnight urinary Ca of < 3.25 mmol/12 h.(ABSTRACT TRUNCATED AT 250 WORDS)