This exploratory study examined relationships between spirituality and immune function in 112 women with metastatic breast cancer. Spirituality was assessed by patient reports of frequency of attendance at religious services and importance of religious or spiritual expression. White blood cell counts, absolute numbers of lymphocytes, T-lymphocyte subsets, and natural killer cells were assessed by flow cytometry. Assessments of natural killer cell activity and delayed-type hypersensitivity responses to skin test antigens provided two measures of functional immunity. In analyses controlling for demographic, disease status, and treatment variables, women who rated spiritual expression as more important had greater numbers of circulating white blood cells and total lymphocyte counts. Upon examination of relationships with lymphocyte subsets, both helper and cytotoxic T-cell counts were greater among women reporting greater spirituality.