Heart and body weights were compared with regard to heart pathology and cause of death in well-defined groups of infants under 1 y of age. In the period 1980 to 1998, out of 468 infants autopsied at the Department of Forensic Medicine in Stockholm, Sweden, 331 died of sudden infant death syndrome (SIDS), while 137 died of other causes. Physical violence was the known cause of death in 30 infants and cardiovascular malformations in another 19. Inflammatory alterations of the myocardium were found in 74 cases (16%): in 17 cases (3.6%) myocarditis was interpreted as the main cause of death; in 45 (10%) it was interpreted as contributing to SIDS and in 12 cases (2.5%) it was observed but judged not to be a contributory cause of death in non-SIDS victims. Two of these infants died as a result of physical violence. Body weight was the best predictor for heart weight as analysed by multiple regression, including age, sex, body weight, length, BMI and birth weight. An equation for estimating heart weight from body weight gave an accuracy within the range 0.75-1.25 in 89.2% and 85.0% of the SIDS and non-SIDS groups, respectively. CONCLUSION: Body weight is the best predictor for estimating heart weight. No evidence supported the notion that heart weight, body weight or birth weight of SIDS victims differs from non-SIDS, although heart weight in infants with cardiovascular malformations deviated from observations in the other groups.