The purpose of this study was to examine the relationship between prevalent radiographic abnormalities of the lumbar spine and the incidence of low back pain in high school rugby players. Three hundred and twenty-seven incoming high school rugby players (15-16 years of age) were recruited between 1986 and 1994. All of them underwent athletic physical examination before participation in high school athletics. The relationship between prevalent radiographic abnormalities of the lumbar spine in the pre-participation athletic physical examination and the incidence of low back pain during a 1-year period after the start of participation in athletics was analyzed. Radiographic abnormalities assessed were spondylolysis, disc space narrowing, spinal instability, Schmorl's node, balloon disc, and spina bifida occulta. The frequency of respective radiographic abnormalities was 15.6%, 13.1%, 32.7%, 14.1%, 11.6%, and 30.6%. Two hundred and forty-three players (74.3%) had at least one of these radiographic abnormalities. The incidence of low back pain was 44.0% in players with no such radiographic abnormalities and 41.2% in those with at least one radiographic abnormality. The incidence of low back pain in players with the respective radiographic abnormalities was 72.5%, 46.5%, 46.7%, 39.1%, 34.2%, and 41.0%, and only players with spondylolysis had a significantly higher incidence of low back pain than those with no radiographic abnormalities. This study shows that a radiographic abnormality, specifically spondylolysis, is a significant radiological risk factor for low back pain in high school rugby players.