BACKGROUND: Minimal flow anesthesia (total inflow rate 0.5 L/min) has been known to increase carbon monoxide production and accumulation in the breathing circuit. We tried to investigate the differences that minimal flow anesthesia shows in the production and elimination of plasma carboxyhemoglobin as compared with high-flow anesthesia. METHODS: Fifty patients were divided into smokers and non-smokers. The smokers were allocated into two groups; high flow (Hs, n = 10) or minimal flow (Ms, n = 13). Non-smoker were also allocated the same way; high flow (Hns, n = 12) or minimal flow (Mns, n = 15). The values (%) of COHb were measured at the preanesthetic baseline, 50 min (30 min after reducing inflow rate in minimal flow), 80 min and 140 min. All values along the time were compared with their baseline within a group. RESULTS: Non-smokers showed a COHb level that didn't change at each time interval as compared with the baseline in group Mns (p = 0.91) and group Hns (p = 0.365). Smokers showed a decreased COHb level at 50 min which increased at 80, 140 min compared with the baseline in group Ms (p = 0.004), but decreased at 50, 80 and 140 min as compared with baseline in group Hs (p < 0.001). CONCLUSIONS: Minimal flow anesthesia does not produce plasma COHb more than high flow anesthesia, but an increase of COHb would be expected in risky groups such as heavy smokers because elimination of carbon monoxide in the breathing circuit was slower than with high flow anesthesia.