IL-10 is a pleiotrophic anti-inflammatory cytokine. Decreased IL-10 expression is associated with an increased breast cancer risk but the mechanism is not clear. This study was designed to test the hypothesis that the loss of IL-10 alters mammary development, even in the absence of inflammation. Wild-type and IL-10-/- mouse littermates were similar in growth, development, and breeding success. Using whole-mounts and paraffin sections, mammary glands from pre-pubertal mice (d21) were found to not be affected by the IL-10 null genotype. However, after the onset of estrous cycling, ductal structure, but not lymph nodes or adipocytes, of IL-10 knockout mice were found to moderately decrease at day 55, 80, and 150 of age. This phenotype was not rescued by lactogenesis. At day 2 of lactation, IL-10 null mice had reduced lobular complexity and glandular area with the retention of adipocytes. These results support the hypothesis that absence of IL-10 reduces glandular development during postnatal development, at maturity, and during the early stages of lactation. Although our study cannot distinguish between a direct IL-10 effect on the epithelial cells and an indirect systemic effect, epithelial cell responses to IL-10 should be considered in the therapeutic applications of cytokines or cytokine ablation.