We conducted a prospective study of non-cardiac surgical patients aged 70 years or more in 23 hospitals in Australia and New Zealand. We studied 4158 consecutive patients of whom 2845 (68%) had pre-existing comorbidities. By day 30, 216 (5%) patients had died, and 835 (20%) suffered complications; 390 (9.4%) patients were admitted to the Intensive Care Unit. Pre-operative factors associated with mortality included: increasing age (80-89 years: OR 2.1 (95% CI 1.6-2.8), p < 0.001; 90+ years: OR 4.0 (95% CI 2.6-6.2), p < 0.001); worsening ASA physical status (ASA 3: OR 3.1 (95% CI 1.8-5.5), p < 0.001; ASA 4: OR 12.4 (95% CI 6.9-22.2), p < 0.001); a pre-operative plasma albumin < 30 g.l⁻¹ (OR: 2.5 (95% CI 1.8-3.5), p < 0.001); and non-scheduled surgery (OR 1.8 (95% CI 1.3-2.5), p < 0.001). Complications associated with mortality included: acute renal impairment (OR 3.3 (95% CI 2.1-5.0), p < 0.001); unplanned Intensive Care Unit admission (OR 3.1 (95% CI 1.9-4.9), p < 0.001); and systemic inflammation (OR 2.5 (95% CI 1.7-3.7), p < 0.001). Patient factors often had a stronger association with mortality than the type of surgery. Strategies are needed to reduce complications and mortality in older surgical patients.