X-linked agammaglobulinemia (XLA) diagnosed in the first year of life is an immunodeficiency with a life-long indication for substitution of immunoglobulins, due to lack of B lymphocytes in the periphery. The decrease of bacterial infection frequency and severity is an effect of immunoglobulin replacement. However, in the majority of patients bronchiectasis and chronic sinusitis with an overgrown mucous membrane develop despite regular substitution. Autoimmune diseases as co-existing diseases in XLA are noted in a few patients presenting symptoms associated with arthritis, scleroderma and myositis. Our patient was diagnosed with XLA in the first year of life, followed by regular substitution of immunoglobulins. The symptoms of pain, edema of muscles of the right shank with skin edema and discoloration after mild injury were noted in a 13-year-old boy. Shulman disease was diagnosed after 6 months of symptoms, based on histopathology of muscle and skin biopsy. Before the diagnosis, non-steroid anti-inflammatory drugs (NSAID) were used with a transient effect. After the diagnosis, therapy included steroids, immunoglobulins in a high dose and immunosuppression, with improvement of clinical symptoms. During methotrexate (MTX) therapy the patient developed two episodes of pneumonia, so mycophenolate mofetil (MMF) was used, with a similar effect. Now, with this therapy, the symptoms are mild and stable without progression.