Immunological status was studied in 15 patients, aged 11 to 71 years, with chronic sclerosing mandibular osteomyelitis. Duration of disease ranged from 1 to 16 years. Routine serological and immunological tests revealed some abnormality in all patients. All except one had an accelerated ESR. The Waaler-Rose titre was raised in 3 patients; in 1 it was 2000 (normal less than 32). Nuclear antibodies were detected in 2 patients, organ-specific antibodies in 3, and antibodies to double-stranded DNA in 1. In only 4 patients were the serum concentrations of immunoglobulins IgG, IgA, IgM and IgE within normal limits; all had normal concentrations of C3 and C4. None had raised antibody titres against viruses or precipitating antibodies to either milk protein or gluten. In none could evidence of food allergy be detected. PHA stimulation of lymphocytes was normal in all patients. HLA-typing in 12 patients showed that 4 had the antigen B13, 3 had B27, and of these 3 patients had the combination B13, B27. The function of neutrophils was studied in 5 patients. In all these patients the chemokinesis was significantly increased, whereas other phagocytic activities were normal. Most of our patients had a hyperactive humoral immune response. The clinical relevance of this finding awaits further, mainly immunogenetic, study.