Four females and two males with seronegative monoarthritis of the manubriosternal joint are reported. Two patients had pustulosis palmoplantaris, one psoriasis vulgaris, and one HLA-B27. The arthritis is considered to be a monoarticular manifestation of seronegative spondylarthritis. Two patients had no other diseases or signs. Four patients were followed for a mean of 8.5 years (range 2-16 years) during which time affection of other joints or the spine was not seen.