The authors study the behaviour of the serum activity of CPK among 22 patients with leptospirosis. Its rise was observed during the acute stage of the disease in close relationship with the lesions of the striated muscle as shown by histological sections studied in 13 cases. The clinicai and histo- logical pictures were related to the CPK activity. The ma in purpose of the present work has been to demonstrate the importance of the CPK serum activity as one more point to complement the diagnosis of leptospirosis, as well its importance as a laboratory aiâ in the differential diagnosis of icterus.