The BB/Wor rat is an animal model of spontaneous autoimmune type I diabetes mellitus. Previous reports have emphasized the presence of lymphopenia in all diabetic and diabetes-prone BB rats. This manuscript reports the appearance of diabetes among three family lines of BB/Wor animals that were previously free of diabetes. Diabetic W-line rats are not lymphopenic and possess normal percentages of phenotypic T cell subsets. Hence the presence of lymphopenia is not obligatory for the occurrence of diabetes in BB/Wor rats.