Primary chronic osteomyelitis (PCO) is a rare non-bacterial autoinflammatory severe disease that most often affects children aged 7 to 12 years and is much less common in adults. The most common areas of lesion are long tubular bones, however, any bones of the skeleton, including the lower jaw, can be affected. The clinical picture of this disease is complex and similar to many tumor and tumor-like bone lesions, which often leads to a significant delay in making a correct diagnosis. The diagnosis of primary chronic osteomyelitis is primarily based on the exclusion of other diseases by comparing the data of radiation diagnosis, clinical course and histological examination of biopsy material. In this article, which is part 1 of a two-part series, the authors review the clinical presentation of PCO, its suggested causes found in the literature, and current diagnostic methods.