OBJECTIVE--To examine the relation between health, behaviour, and superstition surrounding Friday 13th in the United Kingdom. DESIGN--Retrospective study of paired data comparing driving and shopping patterns and accidents. SUBJECTS--Drivers, shoppers, and residents. SETTING--South West Thames region. MAIN OUTCOME MEASURES--Numbers of vehicles on motorways; numbers of shoppers in supermarkets; and hospital admissions due to accidents. RESULTS--There were consistently and significantly fewer vehicles on the southern section of the M25 on Friday the 13th compared with Friday the 6th. The numbers of shoppers were not significantly different on the two days. Admissions due to transport accidents were significantly increased on Friday 13th (total 65 v 45; p < 0.05). CONCLUSIONS--Friday 13th is unlucky for some. The risk of hospital admission as a result of a transport accident may be increased by as much as 52%. Staying at home is recommended.