In a report which highlighted the decreasing deterrent effect of the breathalyser it was suggested that the police should use the breath test as a screen in high-risk situations. Urine and blood alcohol estimations were performed on 59 paired samples which had been collected under conditions that may be found in these situations. The statutory urine/blood conversion factor of 1·33 was applicable in only 2 of these cases. It is concluded that urine alcohol estimation is unreliable in these circumstances.