Factor XIII (FXIII) activity and antigen levels were determined in 955 patients investigated by coronary angiography. Patients were sub-grouped according to the presence or absence of coronary sclerosis (CS+, CS-) and a positive history of myocardial infarction (MI+, MI-). In females, but not in males, adjusted FXIII activity and antigen levels were significantly elevated in the CS+MI+ group compared to in the CS+MI- group. FXIII levels in the upper tertile were associated with significantly increased risk of MI in females, but not in males.