We report a case of diffuse erythrodermic cutaneous mastocytosis with bone marrow infiltration. An 11-month-old female patient was referred to our hospital for intermittent flushing, fever, intense itching, erythematous rash and bullous lesions. Cutaneous biopsy demonstrated diffuse cutaneous mastocytosis. The bone marrow aspirate revealed mast cell infiltration. Ketotifen treatment was very effective.