We have shown previously that a plasma fentanyl concentration of 1.67 ng ml⁻¹ reduced the MAC of isoflurane by 50%. By comparing equal degrees of MAC reduction by sufentanil, we may determine the potency ratio of these opioids. Seventy-six patients were allocated randomly to receive predetermined infusions of sufentanil, and end-tidal concentrations of isoflurane in oxygen. Blood samples were obtained 10 min after the start of the infusion, and just before and after skin incision. Any purposeful movement by the patient was recorded. The MAC reduction of isoflurane produced by sufentanil was obtained using a logistic regression model. A sufentanil plasma concentration of 0.145 ng ml⁻¹ (95% confidence limits 0.04, 0.26 ng ml⁻¹) resulted in a 50% reduction in the MAC of isoflurane. At a plasma concentration greater than 0.5 ng ml⁻¹, sufentanil exhibited a ceiling effect. (Br. J. Anaesth. 1994; 72: 42–46)