Assessing past calcium intake in studies of osteoporosis is of major importance because calcium intake during young adulthood is thought to influence bone mass in later life. However, uncertainty exists about the accuracy of retrospectively reported calcium intake. We therefore evaluated whether a dairy questionnaire estimating calcium intake only from dairy products is a valid method to assess the retrospectively reported calcium intake of 29-year old Dutch males and females 8 and 16 y previously. This was examined in 166 subjects by comparing their original calcium intake reported in 1977 and 1985 by a cross-check dietary history method, with their 1977 and 1985 calcium intake values recalled in 1993 by the dairy questionnaire. The overestimation of the retrospectively reported calcium intake compared with the original intake (11-33%) suggests that the dairy questionnaire is a relatively poor tool to assess the absolute calcium intake of young adults 8 and 16 y ago. Based on the agreement statistics, the dairy questionnaire is a reasonably valid instrument to rank subjects according to their retrospective calcium intake. However, the reporting of the retrospective calcium intake appears to have been influenced by the current calcium intakes reported at the age of 29 y. Nevertheless, the retrospective calcium intake appears to be a better indicator of the original intake than the current intake because the subjects markedly increased their calcium intake over the 16-y period.