The present study was conducted to investigate the effect of dietary protein on urinary Ca excretion, apparent Ca absorption and Ca balance in young and elderly subjects. Young adults (n 29) and elderly persons (n 26) consumed diets containing 12% (diet A) and 21% (diet B) of total energy as protein for 3 weeks according to a randomized crossover design. Results showed no differences between the two age groups with respect to the interaction between protein intake and Ca excretion (both in urine and in faeces), apparent Ca absorption and Ca balance. Therefore analyses were done for both age groups separately and also for the whole group. In elderly persons and in the whole group the Ca excretion in faeces (as a percentage of Ca intake) was lower during the higher protein intake (elderly: diet A, 106 (SEM 7)% diet B, 86 (SEM 7) % P = 0·018; whole group: diet A, 99 (SEM 4) % diet B, 84 (SEM 4) % P = 0·003). In young adults faecal Ca excretion tended to be lower when they consumed diet B (diet A: 94 (SEM 5)% diet B: 83 (SEM 6)% P = 0·093). Relative urinary Ca excretion was greater during the higher protein intake in young adults and in the whole group while relative urinary Ca excretion was not different in the elderly (diet A: 15 (SEM 1) %, 14 (SEM 1) %, 15 (SEM 1) % diet B: 16 (SEM 1) %, 16 (SEM 1) % 17 (SEM 2) % for the whole group, the young and elderly subjects respectively, P = 0·019; P = 0·016; P = 0·243).The resulting Ca balance was not influenced by the amount of protein in the diet in young adults. Values for the elderly and for the whole group showed that the Ca balance during diet A was significantly more negative compared with Ca balance during diet B, despite the higher urinary Ca excretion during diet B. It can be concluded that increasing the protein intake from 12 to 21% of total energy ntake had no negative effect on Ca balance.