Chronic recurrent multifocal osteomyelitis is an idiopathic nonpyogenic autoinflammatory bone disorder involving multiple sites, with clinical progression persisting for more than 6 months and which may have episodes of remission and exacerbation in the long term. It represents up to 2-5% of the cases of osteomyelitis, with an approximate incidence of up to 4/1,000,000 individuals, and average age of disease onset estimated between 8-11 years, predominantly in females. The legs are the most affected, with a predilection for metaphyseal regions along the growth plate. We describe the case of a female patient, aged 2 years and 5 months, with involvement of the left ulna, right jaw and left tibia, showing a predominance of periosteal reaction as main finding.Keywords: osteomyelitis, chronic, multifocal, recurrent, periosteal.case report A female patient aged 2 years and 5 months, being investigated due to pain and swelling in the left forearm for one month. Her parents denied episodes of fever, trauma, and other comorbidities. On physical examination, the child presented normal weight, normal skin color, she was well-hydrated, acyanotic, breathing normally and in good general conditions. Blood counts analyzed in the previous month revealed mild leukocytosis and normocytic normochromic anemia. Levels of C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) were slightly elevated.Computed tomography (CT) of the left forearm without intravenous contrast revealed a periosteal reaction and a small area of loss of cortical compaction, especially in the ulna ( Figure 1A).A biopsy of the left ulna was performed with results describing signs suggestive of chronic osteomyelitis. Culture of the biopsy material did not show growth of microorganisms.Empirical antibiotic therapy was used starting one month after diagnosis, but there was no change in clinical status. A new biopsy was performed in the third month, showing the same findings associated with negative blood culture. Radiographs of the left forearm from the second to the fifth month showed progression of the multilamellar periosteal reaction to a solid type ( Figure 1B).Chest radiography, echocardiography, levels of TSH, T4, IgG, IgA, IgM and complement components were all normal. A hypothesis of chronic recurrent multifocal osteomyelitis (CRMO) was suggested and thus treated with ibuprofen combined with methotrexate, later replaced with sulfasalazine.Between the eighth and ninth month, pain and swelling appeared in the right mandibular region and in the left leg. A facial CT scan showed mainly periosteal reaction at the right mandibular angle with mild bone sclerosis (Figure 2). CT scan of the left leg also characterized a periosteal reaction in the tibial diaphysis with a small area of cortical alteration (Figure 3), similar to that found in the ulna.After 6 months of therapy, there was clinical and general laboratory improvement, with left forearm radiography in the 11 th month showing a regression of the periosteal reaction, despite persistence of corti...