This brief account of dosimetry based on cytogenetic assays reviews the sensitivity of the method and the speed with which a dose estimate can be obtained. The usefulness of chromosome studies to physicians is also discussed. Medical uses include confirming triage category for highly irradiated subjects and providing evidence for nonuniform exposure or other reasons for the presence of surviving functional stem cells. Such information can assist in medical management decisions, especially regarding transplantation of marrow or stem cells and treatment with growth factors. For subjects exposed to zero or low doses, cytogenetic studies can provide useful information for assisting physicians in counseling.