Myocardial infarction is considered the prime cause of death among adult diabetic patients. In a great number of cases, during myocardial infarction the patients don't feel pain or it is atypical. Diagnosis can be neglected, and mortality increases. In search of an explanation for the absence of pain in these patients, the authors studied the autonomic nerve fibers of the heart muscle with argentic and combined techniques, looking for lesions in the sympathetic or parasympathetic nerve fibers that conduct pain. In the five cases of painless myocardial infarction studied, the nerve fibers showed typical lesions of diabetic neuropathy: beaded thickenings, spindle-shaped thickenings, fragmentation of fibers, and diminution of the number of fibers in the nerves. The patients in the control group (five diabetics with painful infarction, five diabetics with infarction, five nondiabetics with painful infarction, and five nondiabetics without infarction) had no lesions. These facts led us to assume that the absence of pain in diabetics with myocardial infarction could be due to a lesion of the afferent nerves that conduct pain.