Synovial fluid and synovial membrane specimens of 11 patients with Chlamydia-induced arthritis (CIA), 24 patients with undifferentiated arthritis (UndA), 4 patients with post-enteritic reactive arthritis, 3 patients with Lyme arthritis and 9 patients with rheumatoid arthritis were investigated for the presence of Chlamydia trachomatis (C. trachomatis). A single stranded DNA-probe was used for nucleic acid hybridization with ribosomal RNA (rRNA) from C. trachomatis. In 4 patients (CIA = 1, UndA = 3) chlamydial rRNA was found in the synovial fluid. In one additional patient (CIA) the specimen of a synovial membrane biopsy was positive for chlamydial rRNA. The detection of intra-articular chlamydial rRNA is discussed as an indicator for the presence of viable Chlamydiae in inflamed joints.