1. Until recently estimates of the urinary excretion of calcium have largely been based upon studies of patients in hospital and, consequently, with changes in the person's diet and physical activity.
2. We have measured the urinary excretion of calcium in seventy-five men and ninety-eight women aged 20–69 years, who were all apparently healthy and who were taking their chosen free diet.
3. For 7 consecutive days each person recorded the weight of all food that was eaten and collected 24 h specimens of urine. We have examined in these persons the relation between the urinary calcium, the dietary intake of calcium, age and sex.
4. 95% of the men excreted less than 400 mg of calcium in the urine per day and 95% of the women excreted less than 340 mg/day.
5. Within the limits of the calcium intake (mean values: men 1148 mg/day, women 1042 mg/day) found in this study, there was no relationship between the dietary calcium and the urinary excretion of calcium except in men over the age of 50 years. Even then the regression accounted for only a minor part of the variation in urine calcium.
6. The urinary excretion of calcium diminished in men over the age of 60 and in women over the age of 50 years.
7. These results are discussed in relation to previous studies of the urinary excretion of calcium. It is concluded that the urinary excretion of calcium is determined more by the absorption of calcium from the intestine than by the intake of calcium, and that the efficiency of absorption decreases in men and women as they grow older.