DNA polymerase θ (Pol θ) is a recently identified family A polymerase that contains an intrinsic helicase domain. Drosophila Pol θ mutants are hypersensitive to bifunctional DNA crosslinking agents and exhibit an elevated frequency of spontaneous chromosomal aberrations, suggesting a role for Pol θ in repair of DNA interstrand crosslinks and in the general maintenance of genome stability. To investigate a possible involvement of Pol θ in tumorigenesis, we have examined its expression in various normal and malignant tissues. Paired tumor and adjacent nontumorous tissues from patients with lung (n = 27), stomach (n = 28) and colon (n = 26) cancer, as well as 26 normal human tissues, were examined for Pol θ expression by RT‐PCR, Northern or Western blot analysis. Pol θ was predominantly expressed in primary lymphoid organs including the fetal liver, thymus and bone marrow where lymphocyte progenitors undergo V(D)J rearrangements of their antigen receptor genes. In addition, Pol θ expression was upregulated in germinal center B cells, in which class switch recombination of the immunoglobulin genes occurs. Examination of Pol θ expression in matched cancer specimens revealed that Pol θ was barely detectable in the nontumorous tissues but was upregulated in 17 of 27 (63%) lung, 11 of 28 (39%) stomach and 20 of 26 (77%) colon cancers. Moreover, patients with high levels of Pol θ expression had a significantly poorer clinical outcome compared with those expressing low levels of Pol θ. These results implicate that Pol θ may have a specialized function in lymphocytes and that its overexpression may contribute to tumor progression. © 2003 Wiley‐Liss, Inc.