We present a sero-epidemiological study of 190 Belgian cases of Lyme borreliosis, a multisystemic disease caused by the spirochaete Borrelia burgdorferi and transmitted by a tick bite. The whole spectrum of clinical pictures was observed in these patients, including "erythema chronicum migrans" (63%), neurological involvement (47%) and arthritis (22%), frequently in combination. Our results are compared to findings in other countries. Among the 437 Ixodes ricinus ticks collected in the Sambre and Meuse valleys around Namur, we discovered 43 ticks (9.8%) with spirochaetes in the midgut. The ecology of these arthropods explains why this infection is more prevalent in the spring and the summer. Perhaps for the same reason, the incidence ranges from low near the coast to medium in the central part and high in the wooded south-eastern part of Belgium. The main conclusions are that Lyme borreliosis is endemic in Belgium, that all the clinical pictures can be observed ant that a clear epidemiological case-definition is needed, combining clinical signs and serological results.