The authors submit an analysis of the clinical pathological material of the nationwide trophoblastic diseases centre (CTN) from 1955-1996. It comprises a total of 5735 cases of trophoblastic disease (TN). This comprises choriocarcinoma (CH) 343 times, so far the largest group of CH verified by histological examination. It comprises furthermore proliferating mole (MP) 202 times complete hydatid mole (MHK) 360 times, partial hydatid mole (MHP) 1150 times persisting trophoblastic invasion (PTI) < 330 times, trophoblastic invasion (TI) 3220 times and persisting trophoblastic disease (PTN) 130 times. The author presents the morphological classification and diagnosis of TN proposed and used in CTN on a nationwide scale. It assessment the importance of different types of TN for their treatment and prognosis. The following units are defined: 1. Trophoblastic invasion, 2. Persisting trophoblastic invasion 3. Partial hydatid mole, 4. Complete hydatid mole, 5. Proliferating mole, 6. Choriocarcinoma which comprises five different types. In trophoblastic invasion the author describes its histological and cytological variability which formerly accounted for as much as 50% false positive diagnoses. Nowadays it is doubtful only in 5%. Persisting trophoblastic invasion was defined in CTN as a new special pathological unit of TN. Usually it recedes spontaneously. Nevertheless in 3% it was in CTN an indication for chemotherapy. In partial hydatid mole and in complete hydatid mole the morphological signs were, defined, which make their differential diagnosis possible which is essential for assessment of their prognosis. After complete hydatid mole choriocarcinoma developed in CTN in 6%. After partial hydatid mole the development of choriocarcinoma was not observed so far in CTN. Proliferating mole is defined in CTN in histological terms which makes its diagnosis from curettage possible. A malignant reversal of proliferating mole was recorded in CTN in 10%. Chemotherapy of proliferating mole was essential in 15%. The mortality rate of choriocarcinoma after proliferating mole declined from the original 85% to 3% and was zero during the last 10 years. According to the CTN classification there are five types of choriocarcinoma which differ markedly as to their biological properties and response to chemotherapy. The histological types of choriocarcinoma were defined on the basis of correlation with orthological trophoblasts of 7 to 20-day-old embryos. The types are: 1. Differentiated syncytiotrophoblastic choriocarcinoma, 2. Mixed differentiated choriocarcinoma, 3. Differentiated cytotrophoblastic choriocarcinoma, 4. Non-differentiated choriocarcinoma, 5. Dissociated choriocarcinoma. Types 1 and 2 respond excellently to chemotherapy and produce high values of hCG. Type 3, 4 and 5 are not very sensitive to chemotherapy or even resistant and produce low values of hCG. Some require primary surgery. They are histologically defined forms of so-called Placental Site Trophoblastic Tumours.