['To evaluate whether GDPs using an index of Orthodontic Treatment Need (IOTN) learning package in a practice setting referred (or treated) more patients with a definite need for orthodontic treatment.', 'A randomised controlled trial.', '20 GDPs (test group) used the package when selecting patients for orthodontic care during normal dental practice. A control group of 20 GDPs did not use the package. Over a 12-month period impressions were taken by the participating GDPs of all patients selected for orthodontic treatment, either referred to another practitioner or to be treated by the GDP.', 'IOTN used by two expert examiners.', 'Among the patients selected for orthodontic treatment by the test group there were more patients with a definite objective need for orthodontic treatment (78.1% versus 62.5% in the control group). After adjustment for potential confounding variables, the odds on a patient selected by a test group dentist being in definite need were 2.24 (95% CI 1.25, 4.01) more than the odds on a patient selected by a control group dentist (P = 0.007).'] Do a field trial of an orthodontic treatment need learning package for general dental practitioners?