A role for interferon-γ (IFN-γ) has been implied in the atherogenic process. To determine whether exogenously administered IFN-γ exerts an effect on the development of atherosclerosis, we intraperitoneally administered either recombinant IFN-γ (100 U/g body weight) or phosphate buffered saline daily for 30 days to atherosclerosis-susceptible apolipoprotein E−/− mice (16-week-old male mice, n = 11 per group) fed a normal diet. Atherosclerotic lesion size was quantified in the ascending aorta. The number of T lymphocytes and major histocompatibility complex (MHC) class II-positive cells within lesions were also quantified in this region. IFN-γ administration reduced serum cholesterol concentrations by 15% (P = 0.02). For both groups, the majority of cholesterol was present in very low density lipoproteins, which were modestly reduced in mice receiving IFN-γ. Despite the decrease in serum cholesterol concentrations, IFN-γ injections significantly increased lesion size twofold compared to controls (119,980 ± 18,536 vs. 59,396 ± 20,017 μm²; P = 0.038). IFN-γ also significantly increased the mean number of T lymphocytes (19 ± 4 vs. 7 ± 1 cells; P = 0.03) and MHC class II-positive cells (10 ± 3 vs. 3 ± 1 cells; P = 0.04) within lesions. These data lend further support to a pro-atherogenic role of IFN-γ.