This paper examines the impact of efficiency on profitability using a panel of 11728 UK manufacturing firms for the period 1993-2007. A key contribution is estimation of the relationship between firm efficiency and profitability in a new way. Part of this novelty involves direct estimation of firm efficiency using a stochastic frontier method rather than inferences being made about the impact of efficiency based on anticipated firm and market behaviour. Two key aspects of the discussion are (1) the shape of the relationship between efficiency and profitability and (2) the way in which this changes in the short and long runs. A simple theoretical model is developed that predicts a 4th order polynomial for efficiency on the right hand side of a profit equation in levels. This model also predicts short-run and long-run impacts that can involve a switching in the sign of the impact of efficiency. Estimation of this model suggests a threshold effect of efficiency on profitability. Below the threshold efficiency has effectively no effect on profitability, but above the threshold the impact is positive in the short-run but negative in the long-run. This switching is consistent with theoretical expectations.