Chronic non-bacterial osteomyelitis (CNO) is a rare autoinflammatory bone disease, affecting mainly children. CNO includes a broad clinical spectrum of symptoms and signs, from mild, limited in time, unifocal osteitis to severe, chronic, active or recurrent, multifocal osteomyelitis. In 2014 diagnostic criteria for CNO were proposed, the Bristol Criteria for the Diagnosis of Chronic Non-bacterial Osteitis, taking into account the clinical picture - location and number of inflammatory foci, characteristic changes on radiological examination (X-ray) and magnetic resonance imaging (MRI), C-reactive protein (CRP) concentration, and changes in bone biopsy. The paper presents the case of a four-year-old boy in whom the diagnosis of multifocal osteomyelitis coexisting with ulcerative colitis was established. Attention was paid to the long diagnostic process of the disease, requiring in the first place differentiation with proliferative diseases. The choice of drugs was also a significant problem in the patient described in view of both intolerance of individual preparations and their ineffectiveness.