Study Design. Exposure discordant twin study. Objective. To investigate the effect of injury on lumbar disc degeneration in monozygotic twins with discordant exposures to recalled previous injury/trauma to the lumbar spine. Summary of Backgrounf Data. Disc degeneration is considered a primary source of low back pain. Despite this, important determinants of disc degeneration other than genotype have not been identified. One possible important determinant of disc degeneration that has undergone limited investigation is previous back injury. Methods. We compared disc degeneration between 37 male monozygotic twin pairs with discordant exposures to recalled previous injury/trauma to the lumbar spine. Data on injury history were obtained through an extensive structured interview. Disc degeneration was assessed using quantitative measures of disc height and disc signal intensity. Results. Disc degeneration did not differ between twins who reported previous back injury and their uninjured cotwins. This finding was consistent for both disc height and disc signal intensity regardless of whether mean scores or greatest difference at any one lumbar level was used in the analysis. Disc height averaged 0.3 mm higher in the injured twin (P = 0.302), and was on average 0.05% higher at the level of the greatest cotwin difference (P = 0.302). There was no evidence that greater period since injury resulted in greater twin differences in disc degeneration. Conclusion. The current study suggests that back injury based on patient report is not an important predictor of future disc degeneration.