These studies of late effects associated with marrow transplant preparative regimens demonstrate the need for continued long-term follow-up of these patients. Sequential studies of growth and development in children demonstrate that few endocrine function abnormalities occur after high dose CY, but multiple endocrine abnormalities occur after TBI containing regimens. Children may have decreased growth rates and abnormal development of secondary teeth. Most young adult patients who received CY only have normal gonadal function recovery and may be fertile. Neuropsychological abnormalities, cataracts and secondary malignancies may also develop. Although the marrow transplant preparative regimen may not be the treatment modality solely responsible for the various observed abnormalities, transplant recipients need to be evaluated carefully for many years to determine onset of abnormalities and to initiate appropriate therapeutic interventions which may improve the quality of life of these unique patients.