PURPOSE: This prospective study examined the association between quadratus lumborum (QL) asymmetry and the development of symptomatic pars interarticularis lesions in the lumbar spine of adolescent cricket fast bowlers. METHODS: Annual magnetic resonance imaging was used to measure QL volume asymmetry and for identifying pars lesions of the lumbar vertebrae in fast bowlers (N = 51) and a control group of swimmers (N = 18). Manual segmentation of axial images spanning the lumbar spine was performed to calculate percent QL asymmetry relative to the bowling- or throwing- (swimmers) arm side. Asymmetry above 100% indicated a larger QL volume on the bowling- (throwing) arm side. RESULTS: The mean QL asymmetry in bowlers of 110.5% (SD = 12.1%) was significantly different from the 96.6% (SD = 5.0%) asymmetry in swimmers (t = 6.75, P ≤ 0.001). In bowlers with symptomatic unilateral L4 pars lesions (N = 11), which all developed opposite the bowling-arm side, the mean 124.3% (SD = 8.3%) QL asymmetry exceeded the 106.7% (SD = 10.1%) asymmetry in bowlers without these one-sided lesions. A logistic regression model (intercept = -22.1, P ≤ 0.001; asymmetry coefficient = 0.18, P ≤ 0.001) demonstrated a significant association between increasing QL asymmetry and L4 lesions in bowlers. Receiver operating characteristic curve analysis demonstrated that QL asymmetry was an excellent predictor of L4 lesions in fast bowlers (area under curve = 0.89, 95% confidence interval: 0.79, 0.98). CONCLUSION: The strong association between QL asymmetry and the development of symptomatic unilateral L4 pars lesions in adolescent bowlers conceivably reflects a mechanical coupling between the loading milieu generating these injuries and preferential hypertrophy of this paraspinal muscle in response to repetitive activation during fast bowling.