BACKGROUND: Precursor B-cell lymphoblastic lymphoma (B-LBL) is an uncommon high-grade neoplasm of immature B cells. In contrast to the more common lymphoblastic lymphoma of T-cell lineage, B-LBL can be an extranodal disease, with a propensity to involve skin and bone. Most reported cases of B-LBL in the skin, a rarity in adults, are manifestations of existing systemic disease. OBSERVATIONS: We report 2 unusual cases of primary cutaneous B-LBL in adults. Fluorescence in situ hybridization studies, not previously reported in primary cutaneous B-LBL to our knowledge, demonstrated rearrangement of the MLL gene in one patient and possible hyperdiploidy in the other, both reported in precursor acute lymphoblastic leukemia. CONCLUSIONS: Review of the literature identified 13 reported cases of B-LBL occurring primarily in the skin, in addition to our 2 cases. Precursor B-cell lymphoblastic lymphoma is more common in children and in young adults, with a tropism for the head and neck region. Histologically, B-LBL must be differentiated from other high-grade lymphoid tumors and small "blue round cell" tumors. Because of the common absence of mature B-cell markers in immunohistochemical studies and the frequent expression of CD99, B-LBL may present a diagnostic challenge. Although there is a suggestion in a limited number of patients that abbreviated therapy may provide long-term disease control, the risk of relapse remains significant, particularly if a patient's condition is misdiagnosed and the patient is treated as having mature B-cell lymphoma. In the absence of prospective studies for this population, patients with B-LBL are treated currently with intensive acute lymphoblastic leukemia regimens.