Objective The validity of the underlying cause of death on death notification forms was assessed by comparing it to the underlying cause determined independently from medical records. Study Design and Setting Retrospective study of 703 deaths in two suburbs of Cape Town, South Africa. Two medical doctors completed a medical review death certificate to validate the registration death certificate for each decedent. Agreement, sensitivity, and positive predictive value were measured for underlying causes of death using the World Health Organization (WHO) mortality tabulation list 1. Results Agreement was poor, with only 55.3% (95% confidence interval [CI]: 51.7, 59.0) of diagnoses matching at WHO mortality tabulation list 1 level. Validity of reported causes of death was poor for HIV, cardiovascular diseases, and diabetes. With correct reporting, the cause-specific mortality fraction for HIV increased from 11.9% to 18.3% (53.6%; 95% CI: 36.9, 77.6), for ischemic heart disease from 3.3% to 7.3% (121.7%; 95% CI: 53.5, 228.7), and for hypertensive diseases from 3.3% to 5.7% (73.9%; 95% CI: 14.4, 167.8). For diabetes, the mortality fraction decreased from 6.0% to 2.3% (?64.3%; 95% CI: ?77.1, ?37.8) and for ill-defined deaths from 7.4% to 2.3% (?69.2%; 95% CI: ?81.0, ?51.6). Conclusion Current cause-specific mortality levels should be cautiously interpreted. Death certification training is required to improve the validity of mortality data.