Reliable questions enhance study design. We assessed the reliability of questions that gather demographic, sun exposure, reproductive history, and physical activity information. Subjects were participants in the Canadian Multicentre Osteoporosis Study (CaMos), a cohort study of Canadian adults recruited January 1996 to September 1997 in nine cities, stratified by sex, age, and location. Following personal interviews, 367 subjects were re-administered part of the questionnaire by telephone. Reliability was assessed using kappa and intra-class correlation. Reliability was excellent for employment status, reproductive history, weight and height (0.91 to 0.97), not differing greatly when stratified by age group or sex. Physical activity and sun exposure were reported with fair to good reliability (0.44 to 0.58), except for moderate activity (kappa = 0.30, 95% confidence interval 0.23, 0.37). Stratification by body mass index did not show significant differences. Many items can be reported reliably, especially those of height, weight, employment status and reproductive history, and, to a lesser extent, physical activity and sun exposure. Similar questions might be used reliably in future studies.