The Welsh Community Diabetic Retinopathy Study was designed to assess the effectiveness of the Field Guide Book for screening for diabetic retinopathy in Europe. A community-based sample (prevalence 2%) of diabetic patients was recruited from four general practices. Standardised training and equipment were provided. All patients were invited to attend practice-based screening sessions on two occasions over 3 years (phases 1 and 2). After mydriasis, clinical ophthalmoscopy was performed by a study optometrist and general practitioners (GPs). 2 x 45 field 35 mm retinal slides were obtained according to EURODIAB protocol. Anonymised slides were assessed by GPs, diabetologists and the optometrist. All the findings were graded externally (reference standard). In phase 2 community optometrists also performed ophthalmoscopy and assessed photographs. For detecting sight threatening diabetic retinopathy using ophthalmoscopy, GPs achieved a sensitivity of 65.7%, specificity 93.8% and positive predictive value (PPV) 65.7%. Community optometrists achieved a sensitivity of 82.2% with a PPV of 50.7%; the study optometrist 79.2 and 55.9%, respectively. The use of 35 mm slides improved sensitivity for the detection of sight threatening retinopathy to 87.3, 91.1 and 97.2% for GPs, community optometrists and the study optometrist, respectively. PPV fell to 51.2% for GPs, 40.6% for community optometrists, but increased to 58.8% for the study optometrist. Diabetologists achieved a sensitivity of 88.7% and a PPV of 65.6%. It is concluded that the European field guide is an effective tool for screening for retinopathy in clinical practice.