The use of seat belts in cars with different seat belt reminder systems was analyzed in a population of 477 injured car drivers, transported by ambulance to Umeå University Hospital after car crashes. The ambulance personnel acted as independent observers regarding the use of seat belts. In this population the non-users were significantly (multivariate logistic regression) fewer (12%) in cars with a combined light and sound reminder, compared to cars without reminder (23%). In cars with only a light signal the proportion of non-users was the same as in cars without any reminder at all. An introduction of more effective reminders, at least in new cars, might be an effective way to increase seat belt usage rates in the long term.