Despite its clinical and histological heterogeneity, anaplastic large cell lymphoma (ALCL) is now a well-recognized clinicopathological entity accounting for 2% of all adult non-Hodgkin's lymphomas (NHL) and about 13% of pediatric NHL. Immunophenotypically, ALCL are of T cell (predominantly) or Null cell type; by definition, cases expressing B cell antigens are officially not included in this entity. The translocation (2;5)(p23;q35) is a recurring abnormality in ALCL; 46% of the ALCL patients bear this signature translocation. This translocation creates a fusion gene composed of nucleophosmin (NPM) and a novel receptor tyrosine kinase gene, named anaplastic lymphoma kinase (ALK). The NPM-ALK chimeric gene encodes a constitutively activated tyrosine kinase that has been shown to be a potent oncogene. The exact pathogenetic mechanisms leading to lymphomagenesis remain elusive; however, the synopsis of evidence obtained to date provides an outline of likely scenarios. Several t(2;5) variants have been described; in some instances, the breakpoints have been cloned and the genes forming a new fusion gene with ALK have been identified: ATIC-ALK, TFG-ALK and TPM3-ALK. Cloning the translocation breakpoint and identifying the ALK and NPM genes provided tools for screening material from patients with ALCL using various approaches at the chromosome, DNA, RNA, or protein level: positive signals in the reverse transcriptase-polymerase chain reaction (RT-PCR) and the immunostaining with anti-ALK monoclonal antibodies (McAb) serve as the most convenient tests for detection of the t(2;5) NPM-ALK since the fusion gene and ALK protein expression do not occur in normal or reactive lymphoid tissue. The wide range of NPM-ALK positivity reported in different series appears to be dependent on the inclusion and selection criteria of the ALCL cases studied. Overall, however, 43% of ALCL cases were NPM-ALK+ (83% of pediatric ALCL vs 31% of adult ALCL). Occasional non-ALCL B cell lymphomas (4%) with diffuse large cell and immunoblastic histology and Hodgkin's disease cases (3%) were NPM-ALK-, but these data are questionable. The aggregate results indicate that, in contrast to primary nodal (systemic) ALCL, the t(2;5) may be present in only 10-20% of primary cutaneous ALCL and rarely, if at all, in lymphomatoid papulosis, a potential precursor lesion; however, these 10-20% positive cases were not confirmed by anti-ALK McAb immunostaining and may represent an overestimate. Positivity for NPM-ALK is associated to various degrees with the following parameters: 44% and 45% of ALCL cases with T cell and Null cell immunophenotype, respectively, are positive, whereas only 8% of cases with a B cell immunoprofile are positive; the mean age of positive patients is significantly younger than that of negative patients; positive cases carry a better overall prognosis (but not in all studies). Recently, the homogenous category of ALK lymphoma ('ALKoma') has emerged as a distinct pathological entity within the heterogenous group of ALCL. The fact that patients with ALK lymphomas experience significantly better overall survival than ALK- ALCL demonstrates further that analysis of ALK expression has important prognostic implications. The term ALK lymphoma signifies a switch in the use of the diagnostic criteria: cases are selected on the basis of a genetic abnormality (the ALK rearrangement), instead of the review of morphological or immunophenotypical features which are clearly more prone to disagreement and controversy. Since its initial description in 1985 ALCL has become one of the best characterized lymphoma entities.