Nuclear DNA measurements on specimens of adenocarcinomas of the lung obtained at surgery from 22 patients showed that in 8 tumors the cells were near diploid and slowly proliferating. These 8 tumors displayed a higher degree of histopathologic differentiation than did the remaining 14 adenocarcinomas. In 3 of the 14 more poorly differentiated tumors the cells were also near diploid but had more S-phase DNA values, indicating increased proliferative activity. In 11 tumors aneuploid DNA stem line or lines were noted. Among patients with slowly growing near-diploid tumors the frequency of tumor recurrence and/or metastases (1 of 8) was lower than among those with more rapidly proliferating near-diploid tumors and aneuploid tumors (9 of 14). Nuclear DNA histogram patterns yielded information about the tumor proliferation activity, modal DNA value and occurrence of aneuploid DNA values. Furthermore, prognostic information about the likelihood of tumor recurrence after surgical treatment was obtained. This information can be utilized in the choice of therapy and in prognostic evaluation.