Diabetes incidence in BB rats is 60-80% and our aim was to investigate whether it is possible to characterize those rats destined to develop diabetes. While the genetic background as well as the environmental factors affecting BB-rat littermates are very similar, body weight reflects some existing variance. The study involved 151 BB rats, and the body weight of each animal was measured daily from birth. Thirty-four animals became diabetic before 100 days of age, and their body weight showed a 5-10% increase compared to the non-diabetic animals for each day of life from day 1 to day 45 (p values 0.0001 to 0.05). This increased body weight in individuals destined for diabetes was seen in both sexes. When investigating whether juvenile body weight has any predictive value, we found that the incidence of diabetes at 100 days of age increased from 22.5% to 46.7% (p<0.01) when the heaviest animals in each litter were selected, Insulin content in pancreas was examined at day 10 and 20, and was found to be significantly reduced in the BB rats with highest body weight compared with rest of the litter (p = 0.02 and p = 0.0005, respectively). The insulin concentration in peripheral blood was significantly reduced in the BB rats with highest body weight at 20 days of age (p = 0.002). When early and late diabetic BB rats were compared at time of diagnosis regarding blood glucose, degree of insulitis and number of small and large islets, no significant differences were found between the groups.