OBJECTIVE To assess the effect of British Summer Time (BST) on road traffic accident casualties and to analyse whether the introduction of year round BST would result in reductions in casualty numbers. METHODS A comparative study of road traffic accident data from before and after the onset of BST. METHODS The county of Cheshire. METHODS Data from a total of 4185 casualties from the period 1983 to 1993. METHODS The effect of BST on both vehicle, cycle, and pedestrian casualties and casualties among schoolchildren. RESULTS The onset of BST in spring was associated with reductions in casualty numbers of 6% in the morning and 11% in the evening. The anticipated rise in casualties with the darker mornings was not seen and as reductions were maximal in the pedestrian (36%), cyclist (11%), and schoolchild (24%) subgroups they were presumed to be due to an altered reliance on vehicular transport. The change back to Greenwich Mean Time (GMT) in autumn produced an anticipated reduction (6%) in casualties in the lighter mornings. The darker evenings, as predicted, were associated with significant increases in casualties (4%), mainly vehicle (5%) and pedestrian (8%) casualties. There was an overall net reduction in casualty numbers when the analysed periods of BST were compared to those during GMT. CONCLUSIONS The use of BST in Cheshire over the period studied was associated with reductions in casualty figures. The application of these results nationally may be expected to produce more and less pronounced changes the further north or south, respectively, the area studied. The introduction of year-long BST would result in beneficial effects on road traffic accident casualties.