Follicular lymphoma (FL) is characterized by t(14;18)(q32;q21), which is the initial genetic perturbation in this disease. Additional genetic mutations are required to generate a fully malignant phenotype. Secondary chromosomal alterations seen in FL include prominent involvement of chromosome 1 in the form of balanced or unbalanced translocations, insertions, deletions, and duplications involving both the p and q arms. We investigated a diagnostically well defined set of 55 t(14;18)-positive FL cases with complex karyotypes by means of multicolor karyotyping. Sixteen cases showed involvement of chromosome 1 and were analyzed in further detail by a novel multicolor banding technique for this chromosome. We defined three groups showing varying complexity of chromosome 1 alterations. The first group revealed simple translocations, such as t(1;2), t(1;6), t(1;8), and t(1;17), involving breakpoints on either the p or the q arm of chromosome 1. The second group showed more complex rearrangements with translocations, insertions, regional duplications, and involvement of more than one partner chromosome with either the p or the q arm of chromosome 1. The third group was defined by highly complex rearrangements involving translocations, regional duplications, amplifications, and intrachromosomal band relocations affecting the entire chromosome 1. All three groups shared interchromosomal rearrangements of chromosome 1 with chromosome 8, often involving the MYC protooncogene site, amplification involving region 1q21–q31, and deletion involving region 1p36. Thus, the use of sophisticated multicolor molecular cytogenetic assays in the investigation of malignant lymphoma allows precise characterization of chromosomal alterations and will provide a better understanding of their biology. © 2002 Wiley-Liss, Inc.