The effects of limited treatment with budesonide, an effective chemopreventive agent, on 5-methylcytosine (5MC) levels, and on the specific hypomethylation and expression of the insulin-like growth factor (IGF) II gene in mouse lung tumors was determined. Female strain A mice were administered two weekly doses of vinylcarbamate intraperitoneally. Thirty-four weeks later, mice were administered dietary budesonide (2.4 or 0.6 ppm in diet) or vehicle control for a period of 7 days and then were killed. We found that the level of 5MC in lung tumors, measured immunologically, was reduced roughly 60% when compared with normal lung parenchyma, which is in agreement with generalized hypomethylation in many tumors. In contrast, tumors treated with budesonide for 7 days exhibited a dose-dependent increase in 5MC, with the higher dose giving 5MC levels similar to control lung parenchyma.