The under-diagnosis and inappropriate treatment of depression in primary care has become a serious public health problem despite the development of many clinical guidelines to guide recognition and treatment of the disorder. This study aims to investigate decision-making processes and to identify factors Which influence general practitioners' (GPs) prescribing decisions and how these factors differ from those the guidelines recommend. Brunswik's lens model, from Social Judgement Theory (SJT); was employed to explore individual treatment decision policies of 40 GPs in the Grampian region of Scotland for 20 case vignettes. These individual policies were then aggregated and compared with those derived from guideline recommendations; important differences emerged between the two in the utilization of cues and there was considerable variation between GPs' policies. Guidelines placed more importance on the duration of symptoms whereas GPs gave weight also to particular symptoms, such as 'thoughts of suicide' and 'sleep disturbance' and patient treatment preference. GPs prescribed antidepressants at a greater rate, than was recommended by the guidelines. The findings have important implications for implementation strategies, which maybe developed to accompany clinical guidelines. Copyright (C) 2002 John Wiley Sons, Ltd.