The sternalis muscle is an unusual variant of the chest wall musculature that can be mistaken for a mass on mammography. We studied the incidence of sternalis muscle in Thai dissected cadavers. Between July 1999 and July 2004, 161 formol-carbol fixed Thai cadavers (49 females, 112 males) were dissected in the anatomical laboratory, Faculty of Science, Prince of Songkla University. Two cases were found to have a sternalis muscle superficial to the pectoralis major muscle in the anterior thoracic wall. The first case, a female, had a unilateral sternalis muscle (type I1), attached to the sternum and pectoral fascia. In the second one, a male, bilateral muscles (type II2) were dissected during a routine dissection. From our findings, there is very low incidence of sternalis muscle of 1.2% in Thais. This rate is the same as has been reported in Taiwan.