In a 5-y retrospective follow-up study in southern Sweden that initially included > 1000 individuals with previously diagnosed erythema migrans, factors that influenced the risk of reinfection with Lyme borreliosis were elucidated. The total frequency of reinfection was 4% and the number of tick bites influenced the risk of reinfection: those bitten > 10 times during the observation period had a 4-8-fold increased risk compared with those bitten < 5 times. Women manifested to a greater extent than men although both genders were bitten equally by ticks, thus indicating that women may be more susceptible to reinfection. All reinfected women were > 44 y old. The county of Kalmar including O ¨ land was found to be highly endemic for reinfection with Lyme borreliosis. Thus the number of tick bites, gender, age and study area influenced the risk of reinfection.