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 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.