Sera from 28 patients with Lyme disease were tested for the presence of anticardiolipin antibodies (ACLA). Seven serum samples had elevated levels of IgM ACLA, and 4 had elevated levels of IgG ACLA. Higher IgM ACLA positivity tended to be associated with neurologic disease, and IgM ACLA levels correlated with the specific IgM response to the infecting spirochete (P less than 0.01). Absorption experiments indicated that ACLA and antispirochete antibodies are largely separate populations. Thus, ACLA may occur in patients with Lyme disease, particularly in those with neurologic abnormalities, and the production of these antibodies seems to be linked to the specific IgM response.