INTRODUCTION The diagnosis codes for sports-related injuries in the Danish National Patient Register (DNPR) are frequently used for research. Even so, their validity remains unknown. The aim of this study was to establish the validity of sports-related diagnosis codes in the DNPR. METHODS The study was conducted as a registry study in the DNPR investigating the diagnosis codes for acute Achilles tendon rupture, Achilles tendinitis, anterior cruciate ligament (ACL) rupture, dislocation of the patella, traumatic tear of the meniscus and degenerative meniscal lesion. For each diagnosis code, patient records were retrieved. We considered a positive predictive value (PPV) of 80% or higher to be satisfying. RESULTS A total of 523 patients were included. The PPV for acute Achilles tendon rupture was 98% (95% confidence interval (CI): 92-100%), for Achilles tendinitis 85% (95% CI: 74-92%), for ACL rupture 96% (95% CI: 88-99%) and for dislocation of the patella 96% (95% CI: 90-99%). Depending on the diagnosis definition used, the PPVs were 56-72% for traumatic tear of the meniscus and 53-77% for degenerative meniscal lesion. CONCLUSIONS This study documented an acceptable validity allowing for epidemiological research of the diagnosis codes for acute Achilles tendon rupture, Achilles tendinitis, ACL rupture and dislocation of the patella. The diagnosis codes for traumatic tear of the meniscus and degenerative meniscal lesion showed a lower validity, and thus caution should be taken when using these codes.