In order to estimate the risk of contracting Lyme disease in the forest areas of north-western Poland, PCR-based studies were carried out on 6,817 Ixodes ricinus ticks for infection by the spirochaete B. burgdorferi sensu lato (s.l.). The studies were performed using the primers for the fla gene, conserved for all European genospecies of B. burgdorferi s.l. Based on the incidence of B. burgdorferi s.l. DNA in I. ricinus ticks at eight sampling sites during 1998-2001, it may be concluded that a risk of contracting Lyme disease is present in the forest areas of north-western Poland. The highest risk of infection (9.4 % of infected ticks) is posed by human contact with female I. ricinus, and the risk is higher in late spring and early summer than in late summer and early autumn. The north-western part of Poland is an endemic region for B. burgdorferi s.l.