Chronic post-traumatic osteomyelitis (OM) represents a particular challenge for nuclear medicine and radiology since clinical and biochemical parameters are frequently unreliable. The aim of this study was to investigate the value of combined bone scan (BS) and immunoscintigraphy (IS) with technetium-99m labelled monoclonal antigranulocyte antibody (MAB) in patients with suspected chronic OM of the appendicular skeleton. Twenty-four patients (17 females and 7 males) with suspected chronic post-traumatic OM were evaluated with three-phase BS/99mTc-MAB-IS. The final diagnosis was established by means of bone culture and histology in 19 cases and clinical follow-up in five cases. The studies were reviewed by two independent and experienced observers; the interobserver agreement was calculated by kappa statistics. The sensitivity, specificity and accuracy of BS alone were 92%, 18% and 58%, respectively. Combined BS/99mTc-MAB-IS had a sensitivity, specificity and accuracy of 84%, 72% and 79%, respectively. Of 24 studies, 11 were true-positive, two false-negative, eight true-negative and three false-positive. Two patients presented with unexpected ectopic haematopoietic bone marrow in the appendicular skeleton that caused false-positive results. A high degree of interobserver agreement was found (kappa=0. 85). It is concluded that combined BS/99mTc-MAB-IS represents a very sensitive and reproducible method with an acceptable specificity for the investigation of chronic OM. Problems may occur in the differentiation of low-grade OM from aseptic inflammation. Another problem is ectopic marrow that may occur in the appendicular skeleton due to a chronic inflammatory stimulus. A former intramedullary intervention in the femur with displacement of haematopoietic marrow may also lead to an ectopic location.