Natural killer (NK) cell activity is impaired in iron-deficient rats. Natural killer cells destroy tumor cells; therefore, iron-deficient rats may be less able to combat cancer growth. Natural killer cell cytotoxicity, both basal and interferon gamma (IFN gamma)-stimulated, was studied in moderately and severely iron-deficient rats challenged with the carcinogen 7,12-dimethylbenz[a]anthracene (DMBA). Female weanling rats were fed ad libitum semipurified diets containing 8, 13 or 42 mg Fe/kg. A pair-fed group was fed the 42 mg Fe/kg diet at the level consumed by the 8 mg Fe/kg group. Following 6 wk of dietary treatment, DMBA-treated rats received a single intragastric dose of DMBA. Dietary treatment was continued. Rats were killed at 1, 4, 8, 14 and 20 wk post-DMBA treatment. Natural killer cell cytotoxicity (both basal and IFN gamma-stimulated) was analyzed. Feeding the 13 mg Fe/kg diet resulted in lower NK cell activity (P = 0.006) and greater tumor burden (P = 0.045) and tumor incidence. Interferon gamma treatment relieved the lower NK cell cytotoxicity observed in moderate iron deficiency. Feeding the 8 mg Fe/kg diet impaired NK cell activity (P = 0.006), but tumor burden and incidence were less than in moderate iron deficiency. In this model, iron deficiency, particularly moderate iron deficiency, contributed to cancer development and compromised NK cell cytotoxicity.