DNA ploidy has been determined by flow cytometry in 28 surgical specimens from patients with adenocarcinoma of the lung (tumor diameter, 3.0 cm or less), and the relationship between DNA ploidy and the clinical stage and survival also has been investigated. The cases were classified according to the DNA index (DI) into group A (DI 1.00 to 1.50), group B (DI 1.51 to 2.00) and group C (DI greater than 2.01). A close relationship was observed between the DI and the survival rates but not between the DI and the clinical stage. Tumor ploidy is considered to be one of the prognostic factors and may be useful in selecting adjuvant therapy.