Humoral immune responses to Borrelia burgdorferi (Bb) have been reported to occur in certain patients with circumscribed scleroderma (CS) (morphoea). Together with the isolation of spirochaetes from CS skin biopsies, this finding was taken to suggest Bb as the aetiological agent of CS. Since there is cellular immunoreactivity to Bb in patients with chronic Lyme borreliosis (LB), Bb-specific lymphocytic responses were tested in patients with CS. For this purpose, peripheral blood mononuclear cells from CS patients and, as controls, from patients with various manifestations of LB, and from healthy volunteers without any evidence of Bb infection, were exposed to Bb organisms for 5 days and then assayed for DNA synthesis. Stimulation indices (SI) > 10 were scored positive. By performing lymphocyte proliferation tests we found: (i) that not only patients with various manifestations of LB but also a considerable percentage of seropositive (five of 13 = 38%) and seronegative (six of 26 = 23%) CS patients exhibit an elevated Bb-induced lymphocyte proliferation; (ii) that the magnitude of the cellular response seen in CS patients is comparable to that encountered in patients with established Bb manifestations; and (iii) that, within a given patient, antibiotic therapy can result in a significant reduction of this response. These results support a causative role of Bb in at least some CS patients. Bb-induced lymphocyte responses were also seen in both seropositive and seronegative erythema chronicum migrans patients. These findings show that the pattern of Bb-specific immune responses is more complex than previously thought, and underscore the importance of lymphocyte function assays in evaluating the diagnosis of potential Bb infection in seronegative patients.