Two population-based case-control studies of lung cancer were conducted on the island of Oahu, HI, between 1979 and 1985. Data from these studies were combined to form the basis of this analysis. Interviews were obtained from 518 men and 230 women with lung cancer and 1,102 male and 524 female controls frequency matched to the cases by sex and five-year age group. The interview consisted of a complete tobacco history, information on body size, and other demographic and life-style information. Weight and Quetelet index (kg/m2) 5 years before diagnosis, but not at 20-29 years of age, were inversely related to the risk of lung cancer among men and women. Cases tended to gain less weight during adulthood than did controls. These associations persisted after adjustment for age, ethnicity, tobacco smoking history, and beta-carotene intake. Our results are consistent with reports by several other investigators of an inverse association between body weight and the risk of lung cancer. However, we were unable to rule out the possibility of bias in our findings due to preclinical disease.