Flow cytometric examinations were performed consecutively on fresh tumor tissue of 90 patients with testicular germ cell tumours. Measured parameters were the DNA index, the stem-cell shoulder fraction (SSF) as an expression of mitotic activity, and the incidence of tumour population above 5c (5c-exceeding events). The results were correlated with the type of tumour, as determined histologically, the local and clinical stage of the tumour, and clinical outcome. In 94.5% of the cases, flow cytometry detected malignant testicular tumour tissue. The median DNA index in the 53 nonseminoma patients was 1.53, in the 37 seminoma patients, 1.82 (P < 0.01). Four out of 42 patients with nonseminomas had progressive disease. These few patients were more likely to have an increase in the S-phase fraction or a positive 5c-ee index than patients without progressive disease. In 24 patients with nonseminoma, it was also possible to examine lymph node tissue. Lymph node metastases were detected in 92.8% of these cases through aneuploidy: the median DNA index was 1.57, which corresponded to that of the primary tumours. This study confirmed the value of flow cytometry for rapid, automatic diagnosis of malignancy in testicular tumours. The method could aid in discriminating between seminomas and nonseminomas.