Unusual presentation of an infected primary hydatid cyst of biceps femoris muscle.

Hydatid cysts of the musculoskeletal system are rare. Unusual magnetic resonance imaging (MRI) findings of an infected primary hydatid cyst of the biceps femoris muscle are presented in a 40-year-old man on hemodialysis for chronic renal failure. No daughter cysts were present within the mother cyst cavity, but there was a fatty nodule which has not previously been described in a muscular hydatid cyst. Although the cyst was infected secondarily, no surrounding soft tissue inflammatory reaction was noted. Hydatid cysts should be included in the differential diagnosis of unusual soft-tissue masses in regions where the disease is endemic.