A 2.8-kilobase major breakpoint region on chromosome segment 18q21 is the site of most t(14;18) translocations typical of human follicular lymphomas. Breaks are focused at the 5′ end of joining (J_H) regions of immunoglobulin (Ig) on chromosome 14, indicating that the translocation occurs at a pre-B-cell stage during attempted heavy (H) chain joining. A new gene from 18q21 (Bcl-2) is placed in the H chain locus creating a unique, translocation-specific J_H;18q21 rearrangement that presumably represents a transformation event. In addition, normal Ig gene joining occurs in a H before light (L) chain and K before λ cascade, creating ordered clonal markers. These serial markers were examined to determine if variations in Ig gene patterns during the natural history of lymphomas represent the emergence of truly separate neoplasms or heterogeneity of a single neoplasm. We examined 45 serial biopsies from 16 B follicular lymphoma patients; six cases showed variation in Ig gene patterns over time. Seven individuals had a detectable J_H;18q21 rearrangement present, and it remained unchanged over 5–10 years. Further rearrangements of H chain genes occurred on the normal chromosome 14 within evolving subclones of the original tumor. λ L chains also underwent additional rearrangements in two instances, while K gene patterns remained unchanged. All variations in the normal H and L chain genes were 2° rearrangements occurring at a mature B-cell stage following the initial successful rearrangement of a H and L chain. In contrast the t(14;18) breakpoint was conserved in each individual, indicating that evolving neoplastic subpopulations arose from a common clonal progenitor cell.