Objective: To assess the use of fibre-optic transillumination (FOTI) as a diagnostic tool in general dental practice. Design: A cross-over quantitative study for the diagnosis of approximal carious lesions by two clinical methods with a qualitative component. Subjects and methods: Seven GDPs were trained to use FOTI as an adjunct to their usual clinical examination to diagnose approximal caries. After 12 weeks of use in their practices four of the GDPs took part in two assessment sessions, set a week apart, using 29 volunteer patients. Each patient was examined on two separate occasions by each GDP using either their standard clinical examination technique alone or supplemented by FOTI examination. The order of the techniques was randomised. Radiographs of each patient were also examined separately. An experienced FOTI user also examined the patients to provide a benchmark. From standard charts the number of enamel and dentinal lesions on approximal surfaces was summed and comparisons made between the techniques. Six GDPs took part in one-to-one interviews. Main outcome measures: Mean numbers of carious lesions recorded by each clinician using each technique. Results: There was a trend for all GDPs to find more enamel and dentinal lesions using FOTI, than with their standard clinical examination with or without radiographs. All GDPs found the FOTI technique a useful adjunct. They used FOTI in different ways and found a variety of uses for it other than caries diagnosis. Conclusions: The FOTI technique increased the detection of approximal carious lesions. It was reported to be a useful diagnostic tool in general dental practice. GDPs and staff enjoyed participating in a research project.