Mantle-cell lymphoma (MCL) is a lymphoproliferative disorder derived from a subset of naive pregerminal center cells characterized by a nodular or diffuse proliferation of atypical lymphoid cells with a monoclonal B-cell phenotype and coexpression of CD5. Two cytologic variants have been identified, typical and blastic. Typical cases show a proliferation of small to intermediately sized lymphoid cells with irregular nuclei and scarce cytoplasm. Blastic variants include a spectrum of intermediate to large cells with round or irregular nuclei and finely dispersed chromatin. These cases have a higher proliferative activity and a more aggressive clinical evolution. MCL is genetically characterized by 11q13 translocations and bcl-1 rearrangement. This alteration leads to a constant overexpression of cyclin D1, which plays an important pathogenetic role, probably deregulating cell-cycle control by overcoming the suppressor effect of retinoblastoma protein (Rb) and p27Kip1. Detection of cyclin D1 may be used as a highly specific marker of MCL because it is expressed in virtually all of these tumors, but in only a few reported cases of aggressive variants of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) and a small percentage of cases of multiple myeloma. Aggressive variants have additional genetic alterations, including inactivation of p53 and p16INK4a tumor-suppressor genes. Clinically, MCL presents in elderly males with advanced disease and frequent extranodal involvement, particularly with involvement of bone marrow, gastrointestinal tract, and spleen. The clinical evolution is relatively aggressive, with poor response to conventional therapeutic regimens and a median survival duration of 3 to 4 years. Further studies are needed to define better new therapeutic strategies for the management of these patients.