Cytophotometric analysis of nuclear DNA in 30 [human] squamous cell carcinomas showed in most tumors a peak value near 2c (diploid) or 3 c (aneuploid), and a wide distribution of other nuclear DNA values. A followup study showed that the prognosis was best for patients with diploid tumor cells and worse for patients with aneuploid tumor cells. The DNA histogram patterns give information about the dominating DNA ploidy of the tumor cells; in some tumors information about the tumor proliferation activity is also provided. These data are useful in prognostic evaluation and may prove useful as adjuncts in the diagnosis and in selection of treatment.