To study the serological response to Borrelia burgdorferi after treatment of late Lyme borreliosis, consecutive serum samples from 20 patients with Borrelia arthritis and 21 with acrodermatitis chronica atrophicans were analysed with capture IgM ELISA and indirect IgG ELISA, both using B. burgdorferi flagella as antigen. Seven patients had positive IgM OD values, whereas all 41 had positive IgG OD values before therapy. In the majority, highly elevated IgG OD values were seen. All patients improved after antibiotic therapy, 32 recovering completely, while 9 had sequelae. At follow-up after 6 months to 5 years, 4/7 patients became negative IgM ELISA, whereas 3 still had slightly elevated IgM OD values 6 months, 1 year and 4.5 years, respectively, after therapy. Only one patient became negative in IgG ELISA during follow-up, although a significant decline in IgG OD values was seen in 22 of the remaining 40 initially IgG-positive patients. The serological response after successful treatment of Borrelia arthritis and acrodermatitis chronica atrophicans may persist for several years even with highly elevated IgG OD values in patients who have recovered completely.