Consistent with experience in other countries, laws requiring motorcyclists to wear helmets led to reduced thefts of motorcycles in Madras, India. This is because many motorcycle thefts are unplanned and anyone riding a stolen motorcycle without a helmet would be highly visible to the police. These findings strengthen the case for the reintroduction of the helmet laws in Madras which were recently repealed after public pressure from riders. They also provide further evidence of the important role played in crime by situational factors. Tables, references