Abstract: Sudden unexpected death is frequent in street heroin addicts. We conducted a histologic study of the sinus node (SN) to offer some evidence about the possible arrhythmogenic cause of death. Postmortem coronary angiography and microscopic examination of the SN and the perinodal area were performed in 50 heroin addicts (group 1) and in 50 nonaddicts (group 2), all men (16–40 years old). In heroin addicts, fatty and/or fibrous tissue replaced SN tissue in 21 cases (42%). Perinodal infiltration was found in 15 cases (30%). Fibromuscular dysplasia in branches of the sinus node artery (SNA) was found in eight cases (16%). Inflammation with focal and/or diffuse concentration of round cells was detected in the SN in 22 cases (44%). Old mural thrombi were also found in 13 cases (26%). The histologic changes in the SN and perinodal area offer an explanation about the possible mechanism of arrhythmia and sudden death in this population.