['Several factors affect the end-tidal carbon dioxide pressure (P(ET)CO(2)) and increase the arterial to end-tidal carbon dioxide pressure gradient (P(a-ET)CO(2)) during general anesthesia. We evaluated the relationship between age and P(a-ET)CO(2) during pneumoperitoneum in the steep Trendelenburg position in patients undergoing robot-assisted laparoscopic prostatectomy (RALP).', 'Ninety-two consecutive patients undergoing RALP were divided by age into a middle-aged (45-65 years) and an elderly (> 65 years) group. Anesthesia was standardized. Heart rate, mean arterial pressure, peak inspiratory pressure, lung compliance, minute ventilation, P(a)O(2), P(ET)CO(2), P(a)CO(2), and P(a-ET)CO(2) were measured 10 min after intubation in the supine position without pneumoperitoneum (T0); and 10 (T1), 60 (T2), and 120 (T3) min after pneumoperitoneum in the Trendelenburg position.', 'Although P(ET)CO(2) did not change significantly during surgery, PaCO(2) and P(a-ET)CO(2) increased gradually with time during pneumoperitoneum in the Trendelenburg position, and both parameters showed greater increases in the elderly than in the middle-aged group. Simple linear regression analyses revealed significant correlations between age and P(a-ET)CO(2) at T0 (P = 0.018), T1 (P = 0.006), T2 (P < 0.001), and T3 (P = 0.001). Linear mixed model analysis showed that P(a-ET)CO(2) was associated statistically significantly with age and duration of pneumoperitoneum in the Trendelenburg position, but age and duration of pneumoperitoneum in the Trendelenburg position were not associated (P = 0.090).'] Does arterial to end-tidal carbon dioxide pressure gradient increase with age in the steep Trendelenburg position with pneumoperitoneum?