The relation of coffee drinking and other behavioral factors to serum gamma-glutamyltransferase (GGT) was examined in 2,494 male self-defense officials aged 48-56 years, who received a retirement health examination at the Self-Defense Forces Fukuoka Hospital between October 1986 and December 1990. Coffee, but not green tea, consumption was inversely related to serum GGT independently of body mass index, alcohol use, and smoking. All of the latter variables were also independently and positively associated with serum GGT. Lower levels of serum GGT associated with coffee drinking were more evident among heavier alcohol drinkers and also among heavier smokers. The findings suggest that coffee may inhibit the inducing effects of alcohol and possibly of smoking upon GGT in the liver.