OBJECTIVES:To compare hip flexor muscle cross-sectional area (CSA) in ballet dancers and athletes, and to evaluate the relationship between hip flexor size and hip pain. STUDY DESIGN:Case control study. SETTING:Elite ballet and sport. PARTICIPANTS:33 professional ballet dancers and 33 age and sex-matched athletes. MAIN OUTCOME MEASURES:CSA's of hip flexor muscles iliopsoas, rectus femoris, sartorius and tensor fascia latae (TFL) on magnetic resonance imaging (MRI) of one hip. Hip pain was scored with the Copenhagen Hip and Groin Outcome Score (HAGOS): HAGOS pain score of 100 was defined as no pain and a score less than 100 was defined as pain. Participants' weight and height. RESULTS:Dancers had larger iliopsoas (P < 0.001, ηp2 = 0.25), TFL (P = 0.001, ηp2 = 0.17), and sartorius (P < 0.001, ηp2 = 0.18) estimated marginal mean muscle CSAs compared to athletes. Rectus femoris muscle size did not differ between groups (P = 0.095, ηp2 = 0.04). Iliopsoas estimated marginal mean muscle CSA was 8% smaller in participants with hip pain compared to those with no hip pain (P = 0.035, ηp2 = 0.7). There was no interaction between the estimated marginal mean muscle CSA of rectus femoris, TFL and sartorius and pain. CONCLUSION:Ballet appears to selectively load iliopsoas, TFL and sartorius muscles but not rectus femoris. Iliopsoas muscle size was smaller in participants with hip pain, but TFL, sartorius and rectus femoris muscle size was not related to hip pain.