Background: Outpatient dietary weight reduction for obesity is unsatisfactory. The objective of this study was to compare the efficacy of an energy prescription diet with usual care (a healthy eating diet) in adult overweight patients referred to a NHS hospital dietetic Out-patient clinic, in terms of weight change over 12 weeks. Methods: Controlled clinical trial (systematic allocation). Results: Of the 53 patients who attended their first appointment, 27 completed the trial. Mean weight loss (kg) after 12 weeks was 4.2 (sd 3.8) on the energy prescription diet (n = 16) and 6.0 (sd 2.8) on the healthy eating diet (n = 11). Conclusions: Patients on a weight reducing diet based on energy prescription or healthy eating lost, on average, clinically significant amounts of body weight by 12 weeks. Mean weight loss was greater by about 50% in the energy prescription group and supports the development of a larger trial to estimate true effect.