Deaths due to external causes require additional medical tests in order to determine the cause of death. If these results are not incorporated into the death register these causes may be misclassified. The objective of this study is to validate the underlying cause of death of the Mortality Register with information obtained from forensic sources in Barcelona between 2004 and 2006.Cross-sectional design. The study population consisted of deceased residents in Barcelona with a medicolegal intervention between 2004 and 2006. The sources of information are the Mortality Registry and the forensic pathology file filled in by Institute of Legal Medicine of Catalonia (ILMC) (gold standard). The study variables are the cause of death, sex and age. Sensitivity and percentage of confirmation (PC) with 95% confidence intervals (95% CI) are calculated.The sensitivity of external causes is 59.7% (95% CI:56.5-62.9) and PC is 96.7% (95% CI:94.8-98.0). Traffic injuries, poisonings and suicides are under-reported in the Mortality Register with a sensitivity lower than 45% and a PC higher than 80%. Symptoms, signs and ill-defined conditions are over-reported with a sensitivity of 89.2% (95% CI:83.4-93.4) and a PC of 28.0% (95% CI:24.2-32.1). There are no differences by sex and age.The validity of the external causes in the Mortality Register is low due to under-reporting and the high proportion of symptoms signs and ill-defined causes. According to the results, incorporating information from forensic sources to the Mortality Register increases the quality of mortality statistics.