A distinctive syndrome of diabetes insipidus developed in four boys in early childhood and progressive spastic cerebellar ataxia developed in adolescence. The boys have been observed for 12 to 19 years and are currently 19 to 25 years old. In patient 1, 12 years after the onset of diabetes insipidus and 4 years after the onset of spastic cerebellar ataxia, bone lesions that proved to be histiocytosis were detected. In patient 2, calcification that developed in the cerebellar dentate nuclei was similar to calcification described in patients with histiocytosis. In the other two patients, an etiologic diagnosis has not been established. We conclude that there is a distinctive syndrome characterized by early diabetes insipidus with subsequent progressive spastic cerebellar ataxia. While histiocytosis may not account for this complex syndrome in all cases, diabetes insipidus followed by progressive spastic cerebellar ataxia merits intensive evaluation.