To estimate the incidence of Lyme borreliosis in France, describe its clinical presentations, and assess its potential risk factors, we conducted a nationwide prospective study in the French Sentinelles Network, consisting of 1178 general practitioners (GPs). Of these, 875 (74%), i.e. 1.6% of all French GPs, participated in the study from May 1999 to April 2000. Eighty-six cases of Lyme disease were reported and validated, of which 77 (90%) consisted of erythema migrans. At national level, the incidence was estimated at 9.4/100,000 inhabitants. Compared to the French general population, Lyme disease patients were older (P<10(-4)), more were living in rural areas (P<10(-3)), and amongst the working population, more were farmers (P<10(-3)) and fewer, salaried workers (P<0.005). Cervidae density correlated strongly with the estimated regional incidence of Lyme disease (r=0.82). Both incidence data and identified risk factors can help to target measures for its prevention and treatment.