A 20-yr-old man with progressive, painful diffuse swelling of his right shoulder muscles is reported. The diagnosis was muscle hypertrophy secondary to a congenital arterio-venous malformation (CAVM). Since a CAVM is not infrequently asymptomatic until the age of 20 to 30 yr and develops insidiously, the diagnosis may be delayed. In our patient, MRI rather than ultrasound and CT, led to the final diagnosis.