Aspirin and other nonsteroidal anti-inflammatory drugs prevent some cases of colon cancer by inhibiting prostaglandin (PG) synthesis. PGE₂ promotes colon neoplasia, as shown by knockout mouse studies on enzymes and receptors in the PG cascade. A few experiments 20 to 30 years ago suggested that PGD₂ may suppress tumors, but a role for biosynthetic enzymes for PGD₂ in tumor development has not been studied. We report here that disruption of the gene for hematopoietic PGD synthase in Apc^Min/+ mice led to ∼50% more intestinal adenomas compared with controls. Tumor size was not affected. By immunohistochemistry, we detected hematopoietic PGD synthase mainly in macrophages and monocytes of the gut mucosa. The mean number of tumors did not increase with knockout of the gene for the lipocalin type of the enzyme, which is not produced in the intestine. On the other hand, Apc^Min/+ mice with transgenic human hematopoietic PGD synthase tended to have 80% fewer intestinal adenomas. The transgene produced high mRNA levels (375-fold over endogenous). There was a suggestion of higher urinary excretion of 11β-PGF_2α and a lower excretion of a PGE₂ metabolite in transgenic mice, but differences (30–40%) were not statistically significant. The results support an interpretation that hematopoietic PGD synthase controls an inhibitory effect on intestinal tumors. Further studies will be needed to prove possible mechanisms, such as routing of PG production away from protumorigenic PGE₂ or inhibition of the nuclear factor-κB cascade by PGD₂ metabolites. [Cancer Res 2007;67(3):881–9]