Forty-six cases of adenocarcinoma of the lung, categorized as stage I (T1N0M0, T2N0M0) pathologically, were studied in terms of nuclear DNA content (NDC) determined by cytofluorometry. They comprised 23 cases each with and without tumor recurrence within five years postoperatively, and the NDC was compared in 23 pairs in which the tumor size and the degree of histologic differentiation were completely matched. The NDC was larger in the recurrent group than in the nonrecurrent group in terms of mean nuclear DNA content (MNDC), DNA histogram pattern, and occurrence of the aneuploid stem cell line, of which the difference in MNDC was statistically significant (p less than 0.05). When the comparison of NDC was made between the nonrecurrent and recurrent groups in the well differentiated (W/D), moderately to poorly differentiated (M.P/D), T1, and T2 subgroups, the NDC of the recurrent group was significantly larger than that of the nonrecurrent group in the W/D (MNDC, p less than 0.01 and DNA histogram pattern, p less than 0.005) and T1 subgroups (MNDC, p less than 0.05), but not in the M.P/D and T2 subgroups. The results indicate that the NDC is a good predictor of prognosis in stage I adenocarcinoma of the lung and its prognostic importance increases in more differentiated and smaller tumors, compared to less differentiated and larger tumors.