Two sisters with diffuse chronic sclerosing osteomyelitis of the mandible and the humerus and the synovitis, acne, pustolosis, hyperostosis and osteitis syndrome (SAPHO syndrome) are presented. The diagnoses of diffuse chronic sclerosing osteomyelitis at the age of 12 years and 27 years, respectively, were based on typical medical history, clinical symptoms and radiographic, histologic and scintigraphic findings. Because skin lesions and scintigraphic enhancement of the sternoclavicular joints with hyperostosis were present, a SAPHO syndrome was diagnosed in both sisters. Microbiological cultures of biopsy specimens revealed coagulase-negative Staphylococcus aureus at the humerus and Haemophilus parainfluenzae, Streptococcus, Actinomyces and Veilonella species at the mandible. Repeated operative procedures, including decortications, resection and reconstruction, and multiple histologic and microbiologic studies were performed over a period of up to 20 years. Since HLA typing yielded identical gene loci, we suggest that hereditary and autoimmune factors may play a role in the pathogenesis of these cases.