Investigation of subclinical infections with Borrelia burgdorferi in risk groups in Switzerland, Austria and southern Germany showed an infection rate of 16%-27%. Random samples of patients admitted to the neurological department of a hospital on the outskirts of Hamburg gave the following results: in 100 sera there was in 27% a positive Elisa-test, in 77 sera the IFT-test was positive in 13%, and in 210 sera 6% showed a positive IFT-test after absorption; the IFT- and IFT-Abs. were compared in 15 sera. The investigation confirms that subclinical infection with Borrelia burgdorferi in northern Germany is not uncommon. The fact that increased levels of IgG and IgM antibodies do not necessarily mean the presence of an acute or chronic disease should be remembered in making decisions about treatment.