In normal subjects, alcohol increases handwriting size, but the mechanism is not understood. Here we show that the alcohol effect on handwriting can be explained by a selective impairment of kinaesthetic perception. Thirty volunteers (15 male, aged 18-29 years) took part in an open study. They were tested before and after a drink containing vodka intended to produce a blood alcohol concentration of about 80mg/100ml. Tests included kinaesthetic distance estimation, in which volunteers worked with preferred hand and arm behind a screen which hid their movements; visual distance estimation; and measures of handwriting and drawing. Blood alcohol concentration at 55min, based on breathalyser measurements, was 76.7mg/100ml (SD 9.8). When asked to move the hand and mark a distance of 10cm from a starting point, distances estimates increased by 7-10% (p 0.01). Similar increases were seen for writing words and drawing characters. Signatures were increased in height but not in length. Distances estimated visually were increased much less, by 3-4% (p 0.05). Tests of psychomotor performance indicated the expected effects of ethanol. These results suggest that ethanol affects writing size by reducing kinaesthetically perceived distances.