Seventy-three patients with undiagnosed arthritis of undetermined aetiology, 94 patients with classified arthritis (rheumatoid arthritis, ankylosing spondylitis, etc.) and 70 controls were studied for clinical and serological manifestations of Lyme borreliosis. The patients were recruited from the three rheumatology units in the most southern part of The Netherlands. A clinical diagnosis of possible Lyme borreliosis was made in seven of 73 patients with arthritis of undetermined aetiology, in four of 94 patients with classified arthritis and in one of the controls. A definite diagnosis of Lyme borreliosis could be made in only one patient who belonged to the arthritis of undetermined aetiology group. This patient had erythema migrans, arthritis of the knee joint and showed positive antibodies to B. burgdorferi . In the southern part of The Netherlands, Lyme arthritis does not seem to be a frequent cause of arthritis of undetermined aetiology.