This prospective study compared ultrasound measurement of the lumbar vertebral canal with subsequent back pain; 669 mining and nursing recruits had been measured before 1980 and 450 replied to a postal questionnaire in 1990. There was no significant difference in the prevalence of back pain or the need to be off work with back pain, in those whose canals were above and below the mean. However, going to bed with back pain was more frequent in those with small canals and approached significance (P = 0.068). Those with small canals were more likely to visit doctors and have treatment for back pain. Canal measurement is not a predictor for back pain, but as a risk factor for severe back pain in early working life, it approaches significance.