BACKGROUND This study investigated the reliability and the validity of the QUANTAP (QUANTification de l'Activité Physique) interview-administered survey, a new computer-assisted tool designed to determine physical activity over a lifetime. METHODS The tool was used to assess lifetime exercise habits in four dimensions (sport at school, leisure sport, occupation, daily activities) in 419 men and women aged 13-90 years. Physical activity indicators (time spent and energy expenditure) were calculated for 20-year periods. The inter-observer and intra-observer reliability of the tool was studied in two subgroups of 30 subjects. RESULTS Intraclass correlation coefficients for intra-observer and inter-observer reliability varied from 0.56 to 0.96 and from 0.42 to 0.99 respectively according to the dimensions and indicators considered. Energy expenditure was not statistically significantly different from recommended nutritional intake in either males or females. Percent body fat at the time of the survey correlated with leisure sport (particularly in recent periods of practice): age-adjusted correlation coefficients varied from - 0.23 to - 0.45 among males, and from - 0.19 to - 0.31 among females. CONCLUSION The results indicate that QUANTAP is reliable and valid to assess lifetime physical activity. It therefore provides a tool with which the long-term effects of physical activity on current health may be studied.