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Background Acute kidney injury (AKI) is a key complication of rhabdomyolysis. Creatine kinase (CK), the standard muscle injury biomarker, has limited predictive value. Myoglobin may be more accurate but has its limitations. The myoglobin-to-CK ratio may be a stronger predictor but lacks prospective validation. We aimed to validate this ratio using admission values, compare it with CK and myoglobin, and identify thresholds for early AKI risk stratification.Methods Prospective multicentre cohort study across four hospitals in Oslo and Akershus, Norway (2019–2022). All adults with CK ≥5000 U/L and/or myoglobin ≥1000 ng/mL at emergency department presentation or within 72 hours thereafter were eligible, irrespective of admission. Primary outcome was AKI. Predictive performance of CK, myoglobin and the myoglobin-to-CK ratio was assessed using logistic regression and receiver operating characteristic (ROC) analysis.Results In the 310 included patients, 108 (35%) developed AKI. In ROC analysis the myoglobin-to-CK ratio was the strongest predictor of AKI on admission (area under the curve (AUC) 0.84, 95% CI 0.79 to 0.89), followed by myoglobin (AUC 0.73, 95% CI 0.67 to 0.79); CK was a weak predictor (AUC 0.31, 95% CI 0.25 to 0.37). In quartile analysis, 72% of patients in the highest ratio quartile developed AKI, with a ratio ≥0.20 indicating risk. For myoglobin, 65% in the highest quartile developed AKI, with values ≥6372 ng/mL indicating risk. ROC analysis identified optimised cut-offs of 0.48 for the ratio (sensitivity/specificity 79/82%) and 4489 ng/mL for myoglobin (61/76%). Combined, these cut-offs increased sensitivity and negative predictive value to 89% (95% CI 81.3% to 94.4%) and 92% (95% CI 86.2% to 96.0%), respectively, suggesting potential for identification of both high-risk and low-risk patients at admission.Conclusions The myoglobin-to-CK ratio was the strongest early predictor of AKI. Combined with myoglobin ≥4489 ng/mL, a ratio ≥0.48 identified 89% of AKI cases and ruled out 92% of non-AKI cases, suggesting a potential admission-based tool for early AKI risk stratification in rhabdomyolysis.Trial registration number NCT04118608.