This is a retrospective study of 13 patients with muscular hydatidosis--i.e., 4% of the 309 cases of hydatid disease treated in our department during 1983-1999. The commonest clinical finding was an asymptomatic and slowly growing mass (7). Puncture or incision of the mass was followed by an infection of the cystic cavity with fistulization in 2 patients. The immunological findings were false negative in 4 patients. MR images were obtained in 4 patients before diagnosis, and were highly suggetive of hydatid disease. The cystic cavities in all 9 patients subjected to radical surgery healed without chemotherapy. Radical surgery was not possible in 4 cases, in 3 of whom the sacrum was involved. Medical treatment of these patients did not eliminate the disease and new operations were necessary.