Because diabetes incidence (in the context of animal diabetes defined as percentage of animals developing diabetes until the age of 200 days) in BB rats is largely the same in males and females, a sex-influenced or sex-linked inheritance of diabetes has been excluded. However, in crossing studies using diabetic rats of the BB/OK rat subline and different diabetes-resistant rat strains, an excess of females was frequently observed, generally connected with an excess of diabetic males. Therefore the incidence of diabetes in 12 litters of [(dBB x DA)F1 x dBB] first backcross hybrids (BC1) and in 331 litters of BB/OK rats was analyzed according to sex ratio of litters and phenotype of parents (d-diabetic, nd-nondiabetic). In BC1 hybrids and BB/OK rats, about 50% of litters with an excess of females were observed, but an excess of diabetic males was established only in BC1 hybrids. However, in BB/OK rats correlations were found between diabetes incidence and litters with 1:1 proportion of diabetic males to females (r = -0.42 p < 0.05), phenotype of parents d x d (r = 0.73 p < 0.01) and nd x nd (r = -0.69 p < 0.01). Furthermore, an indirect influence is observable from correlations between the phenotype of parents d x d and litters with an excess of males (r = -0.40 p < 0.05) and diabetic females (r = -0.42 p < 0.05) as well as litters with a 1:1 proportion of diabetic males and females (r = -0.42 p < 0.05). It is concluded that diabetes in BB/OK rats is most probably incompatible with a "biologically normal" sex proportion in litters and that a sex-related inheritance appears to exist.