In the first five-year period of the Danish Testicular Carcinoma Study (DATECA) 1058 consecutive testicular germ cell tumours were examined. Of these, 554 were seminomas comprising 515 of typical type, 26 anaplastic and 13 spermatocytic; 497 were non-seminomas comprising 145 pure tumours and 352 mixed tumours of various types. Among the various subtypes of non-seminomas embryonal carcinoma (EC) was recorded in 87 per cent, endodermal sinus tumour (yolk sac tumour; EST) in 22 per cent, teratoma (T) in 55 per cent and choriocarcinoma (CC) in 17 per cent. Only very few tumours were pure EST or pure CC. Five tumours were recorded as 'others or uncertain'. The tumours were graded with regard to various histologic features. Moderate and severe necrosis, bleeding, and a large number of mitoses were significantly more frequent in non-seminomas. The presence of tumour tissue at the resection margin was also more frequent in non-seminomas. Tumours with a largest diameter of less than 2.5 cm had already caused metastases in 16 per cent of the seminomas and 29 per cent of the non-seminomas. Increasing size of the tumours was associated with increasing frequency of metastatic disease but this association was not directly proportional. Distribution of the various histologic types according to the stage of disease varied. Thus, 78 per cent of the seminomas presented in stage I while 54 per cent of the non-seminomas had localized disease. Anaplastic seminomas were distributed similarly to the non-seminomas while all spermatocytic seminomas, with one exception, were recorded as stage I. Of non-seminomatous subtypes pure EC was associated with the highest frequency of stage III, followed by mixed tumours containing CC components. Although the present series is large the heterogeneity of germ cell tumours demands further investigation of larger numbers to confirm some of the findings.