Modal DNA values of tumours from various sites may exhibit (1) a diploid-near diploid distribution, (2) an exponential distribution in the tetraploid to triploid range, or (3) a log-normal distribution in the triploid to tetraploid range. Examples of these various types of distribution are non-Hodgkin's lymphoma (1), aneuploid prostate carcinoma (2), aneuploid colon, breast, cervix and testicular carcinomas (3). These differences indicate dissimilarities in tumour development. In aneuploid tumours from the same site both tetraploid exponential and triploid-tetraploid log-normal distributions may occur. In bladder carcinoma these are related to grade. Modal DNA values in tumours are 10% higher than would be expected from modal chromosome numbers. This difference seems not to be due to a relative increase in large-sized chromosomes or due to technical shortcomings. Chromosome studies also show the possibility of the existence of near diploid malignant cells in grossly aneuploid tumours. Modal DNA values are connected with functional tumour properties by the proportion of S-phase cells. The significance of the latter is exemplified by follow-up of patients with bladder and cervix carcinoma.