We developed a risk score for 30-day postoperative mortality: the Perioperative Mortality risk score. We used a derivation cohort from a previous study of surgical patients aged 70 years or more at three large metropolitan teaching hospitals, using the significant risk factors for 30-day mortality from multivariate analysis. We summed the risk score for each of six factors creating an overall Perioperative Mortality score. We included 1012 patients and the 30-day mortality was 6%. The three preoperative factors and risk scores were ("three A's"): 1) age, years: 70 to 79 = 1, 80 to 89 = 3, 90+ = 6; 2) ASA physical status: ASA I or II = 0, ASA III = 3, ASA IV = 6, ASA V = 15; and 3) preoperative albumin < 30 g/l = 2.5. The three postoperative factors and risk scores were ("three I's") 1) unplanned intensive care unit admission = 4.0; 2) systemic inflammation = 3; and 3) acute renal impairment = 2.5. Scores and mortality were: < 5 = 1%, 5 to 9.5 = 7% and > or = 10 = 26%. We also used a preliminary validation cohort of 256 patients from a regional hospital. The area under the receiver operating characteristic curve (C-statistic) for the derivation cohort was 0.80 (95% CI 0.74 to 0.86) similar to the validation C-statistic: 0.79 (95% CI 0.70 to 0.88), P = 0.88. The Hosmer-Lemeshow test (P = 0.35) indicated good calibration in the validation cohort. The Perioperative Mortality score is straightforward and may assist progressive risk assessment and management during the perioperative period. Risk associated with surgical complexity and urgency could be added to this baseline patient factor Perioperative Mortality score.