A 26-year-old female patient was admitted to the hospital because of bilateral gonarthritis, lymphadenopathy and disseminated chorioiditis as primary manifestation of Lyme borreliosis. Antibody titers against Borrelia burgdorferi did not reach diagnostic levels as determined by an indirect immunofluorescence assay. However, diagnosis was established by the detection of IgG and IgM antibodies in Western blot analysis, and by the demonstration of an enhanced T-cell proliferation to Borrelia burgdorferi in a lymphocyte proliferation assay. This case report indicates that arthritides may already occur in stage II (disseminated infection) of Lyme borreliosis. Therefore, Lyme borreliosis must be considered in patients with chorioiditis and pauciarticular arthritis. In the case of non-diagnostic antibody titers in indirect immunofluorescence tests (or ELISA), Western blot analysis and lymphocyte proliferation assays should be performed in addition.