OBJECTIVE: The objective of the study was to determine the magnitude and side of quadratus lumborum (QL) asymmetries in elite, adult, cricket fast bowlers and the relationship with lumbar spine injury. DESIGN: Cohort study. SETTING: Cricket fast bowers had magnetic resonance (MR) scans at the start of a cricket season and their injury characteristics over the next cricket season were compared with the amount of QL asymmetry. PARTICIPANTS: Twenty-three elite, asymptomatic, adult, cricket fast bowlers. ASSESSMENT OF RISK FACTORS: The cross-sectional area (CSA) of QL was measured using MR imaging. The association between side-to-side differences in CSA (asymmetry) was evaluated as a possible risk factor for development of lumbar spine injury. MAIN OUTCOME MEASURES: The main outcome measurements were QL CSA and asymmetry in relation to lumbar spine injury in cricket fast bowlers. RESULTS: There were a greater proportion of dominant- (bowling arm) side asymmetries (65%). Asymmetry magnitudes that favored the dominant side were not significantly larger than those on the nondominant side. Four participants who had bone oedema on MR imaging preseason went on to develop symptomatic lumbar stress fractures. Participants with no lumbar spine injury had significantly larger QL asymmetries than those who sustained lumbar spine injury. CONCLUSIONS: Cricket fast bowlers demonstrated asymmetrical QL development, which may be related to the trunk positions adopted in the fast bowling technique. Uninjured bowlers had larger asymmetries than those who developed lumbar spine injury, which is contrary to some previous research.