A questionnaire, a self-administered log, and an observational method were simultaneously applied in the workplace of 35 mechanical repair men to assess exposure to strenuous postures and movements of the back. The average duration of time spent in a standing position was considerably underrated by the workers, while the duration of sitting was strongly overrated when compared with the ratings obtained with the observational method. The workers' ratings of duration with a bent or rotated trunk was two to four times lower than the observed duration. The estimate of the average number of lifts performed per hour was more than four times higher in the questionnaire than in the log. The same striking difference was found for the frequency of bending or rotating of the trunk. These results suggest that the reliability of questionnaire methods for the assessment of postural load in epidemiologic studies is probably not very high.