Exposures early in life seem to play an important role in the development of gastric cancer, but their nature is not well understood. In a population-based case-control study, we examined weight, height and body-mass index (BMI) at the age of 20 as well as indices of socioeconomic conditions. Face-to-face interviews were conducted with 338 of 456 eligible histologically confirmed gastric cancer patients and 679 of 880 eligible control subjects, sampled from population registers and frequency matched by age and gender. Gastric cancer risk was negatively associated with height. Risk was positively, associated with weight at age 20 in both sexes. The highest BMI-quartile was associated with an increased risk. This association between BMI and risk was confined to BMI at age 20, and disappeared for BMI 20 years prior to interview. High socioeconomic status (SES) as well as long education carried a decreased risk of gastric cancer. Number of siblings was positively associated with risk. Factors related to short stature and high adolescent body weight are associated with an increased risk of gastric cancer, as is a high number of siblings. These factors may reflect influences of dietary patterns early in life.