Interphase cytogenetics is still in its infancy but the information which it is capable of providing will lead to a greater understanding not only of the normal interphase nucleus but also of the genetic content of tumor cells and will facilitate antenatal diagnosis of some hereditary diseases. Application to tumors will provide the ability to correlate chromosome complement (and ultimately single gene content) with tissue morphology and clinical tumor behaviour, perhaps providing prognostic information. We anticipate that this approach will give clues to consistent genetic abnormalities within tumors which can unambiguously be assigned to malignant as opposed to the normal stromal cell content of the tumor.