||Few empirical studies of supply response using the profit function have accounted for technical inefficiency. Using farm-level panel data from Ethiopia, this study examines the effect of incorporating technical inefficiency in estimating the supply response of peasant farmers. Two systems of output supply and input demand equations are estimated and compared: the conventional model in which technical efficiency is assumed and another in which technical inefficiency is explicitly incorporated. The model with technical inefficiency is preferred on grounds of theoretical consistency and improved estimates, although model comparison tests are not conclusive. Incorporation of inefficiency generally increases the magnitudes and the statistical significance of own price elasticities, substantially so in the case of fertilizer and fertilizer-intensive crops, and alters the priority attached to nonprice factors. An important result is that only the specification with inefficiency reveals a significant effect of access to extension services on output. Only this specification finds that output increases with household size, which one expects as the farms in the sample are largely subsistence and producing for own consumption. Furthermore, the results demonstrate that farmers’ response to incentives is considerably restricted by inefficiency, suggesting that the traditional model would overstate response by excluding the efficiency variable.