Unthrifty calves occurred sporadically in Japanese Black (beef cattle) in an area in northeastern Japan. The states of unthrifty development, pedigree, clinico-biochemistry and the secretory function of bovine growth hormone (bGH) in pituitary were investigated. The total cholesterol concentration and CK, AST and LDH activities in the serum showed higher values than those of control calves. Basal bGH concentrations in the serum and bGH secretory reactivity in the insulin tolerance test (ITT) were showed to be significantly lower than those of the control calves. Furthermore, sperm donated from a specific bull had been used for these unthrifty calves. This study suggested that the present occurrence of unthrifty calves represented ateliosis possibly caused by congenital hypopituitarism which decreased of bGH secretory function.