Immunophenotypes in lymphoproliferative diseases (LPD) are prognostically significant, yet causative factors for these conditions, and specifically those associated with heritable risk, remain elusive. The full spectrum of LPD seen in humans occurs in dogs, but the incidence and lifetime risk of naturally occurring LPD differs among dog breeds. Taking advantage of the limited genetic heterogeneity that exists within dog breeds, we tested the hypothesis that the prevalence of LPD immunophenotypes would differ among different breeds. The sample population included 1,263 dogs representing 87 breeds. Immunophenotype was determined by the presence of clonal rearrangements of immunoglobulin heavy chain or T-cell receptor γ chain. The probability of observing the number of B-cell or T-cell tumors in a particular breed or breed group was compared with three reference populations. Significance was computed using χ2 test, and logistic regression was used to confirm binomial predictions. The data show that, among 87 breeds tested, 15 showed significant differences from the prevalence of LPD immunophenotypes seen across the dog population as a whole. More significantly, elevated risk for T-cell LPD seems to have arisen ancestrally and is retained in related breed groups, whereas increased risk for B-cell disease may stem from different risk factors, or combinations of risk factors, arising during the process of breed derivation and selection. The data show that domestic dogs provide a unique and valuable resource to define factors that mediate risk as well as genes involved in the initiation of B-cell and T-cell LPD. ©2005 American Association for Cancer Research.