Relatively little is known about the factors in Canada which lead to osteoporosis and its concomitant fractures. The Canadian Multicentre Osteoporosis Study (CaMos) is a prospective cohort study which will estimate the incidence and prevalence of declining bone mass and fractures. The impact of osteoporosis in Canada will be assessed, including regional variation and the effect of various risk factors. The study will provide information for developing prevention programs. The cohort has been drawn from a random population-based sample of non-institutionalized men and women 25 years old or more and living within 50 km. of nine cities in Canada. Through telephone interviews 9,423 participants have been recruited. All answered an extensive questionnaire, and had physical measurements related to bone quality taken. This paper details the CaMos baseline and five-year follow-up protocol.