To compare hip joint effusion-synovitis prevalence in professional ballet dancers with nondancing athletes and to evaluate the relationship between effusion-synovitis and clinical measures and cartilage defects.Case-control study.Elite ballet and sport.Forty-nine professional ballet dancers and 49 age-matched and sex-matched athletes.Group (dancers/athletes), sex, age, years of training, Copenhagen Hip and Groin Outcome Scores (HAGOSs), hip rotation range of motion (ROM), generalized joint hypermobility (GJH), and hip cartilage defect scores.Hip joint effusion-synovitis (absent, grade 1 = 2-4 mm, grade 2 = >4 mm) scored with 3-Tesla magnetic resonance imaging.Hip joint effusion-synovitis was found in 22 (45%) dancers and 13 (26.5%) athletes (P = 0.06). Grade 2 effusion-synovitis was only found in dancers (n = 8, r = 0.31, P = 0.009). The prevalence of effusion-synovitis was similar in men (n = 11, 26%) and women (n = 24, 43%, P = 0.09). Female dancers with effusion-synovitis had lower HAGOS pain (r = 0.63, P = 0.001) and sports/recreation scores (r = 0.66, P = 0.001) compared with those without effusion-synovitis. The HAGOS scores were not related to effusion-synovitis in male dancers or female and male athletes (P > 0.01 for all). Effusion-synovitis was not related to hip ROM, GJH, or cartilage defect scores (P > 0.05 for all).Hip joint effusion-synovitis was related to higher levels of pain and lower sports/recreation function in female ballet dancers. Effusion-synovitis was not related to hip rotation ROM, GJH or cartilage defects. Larger sized joint effusion-synovitis was exclusively found in dancers.