Systemic mastocytosis (SM) is characterized by pathologic expansion and activation of mast cells. The main clinical manifestations of SM include skin involvement, gastrointestinal symptoms and anaphylaxis due to the release of its mediators. Thirty percent of pat ients with SM have a low bone mass and 20% fractures. At the same time, SM affects 10% of male patients with idiopathic osteoporosis. Measuring serum tryptase is essential for the screening of MS. We report two cases of SM with bone involvement. A 25-year- old woman with prior diagnosis of SM, based on skin involvement, flushing, high serum tryptase and compatible bone marrow (BM) biopsy and genetic study. Low bone mass was diagnosed and treatment was started with calcium and vitamin D plus oral bisphosphona tes with adequate response. A 47 years old man who presented with multiple osteoporotic vertebral fractures and low bone mass. Treatment with vitamin D and alendronate was started, but the patient developed new vertebral fractures. The study was extended w ith measurement of serum tryptase that was elevated. Diagnosis of SM was confirmed with BM biopsy and the patient was referred to hematology for specific care. These cases emphasize the importance of bone assessment in SM, as well as the need to rule out S M in patients with osteoporosis and no evident cause.