Natural abundance 13C NMR was used to determine relative amounts of fatty acid subclasses present in fibroadipose tissue from the human breast in healthy and cancer patients and in breast carcinoma tissue. Resonances corresponding to the carbon atoms of triacylglycerides were obtained when adipose tissue constituted more than 10% of the carcinoma. Resonances corresponding to phospholipids and proteins were also observed when the percentage of adipose tissue was lower. No significant difference between the levels of unsaturated fatty acids in adipose tissue from cancer and non-cancer patients was found. However, significant differences in the levels of monounsaturated and saturated fatty acids of carcinoma compared to non-cancerous tissue was found, as was a nearly significant difference for the levels of polyunsaturated fatty acids in these two tissue types. These findings suggest an alteration of cellular lipid composition in neoplastic mammary tissue.