Serum samples from 43 patients with positive test for syphilis only in the FTA-Abs test, were evaluated. Three had primary or treated syphilis. Twenty-one (49%) had clinical and/or serological signs of Lyme borreliosis as assessed by whole-cell sonicate Borrelia burgdorferi ELISA and Western blot techniques. Seven (16%) had genital Herpes simplex infection and the remaining 12 patients, miscellaneous disorders. In control sera from 30 patients with Lyme borreliosis an isolated positive FTA-Abs reaction was found in 13 patients (43%). Elevated Borrelia ELISA titres were found in nine of 30 (30%) syphilitic patient serum samples, whereas Western blots for Borrelia were negative. Six per cent of healthy blood donors were seropositive for Borrelia. Lyme borreliosis is an important cause of cross-reactions in the FTA-Abs test. Other serological tests for syphilis and Western blot for Borrelia are useful for discrimination.