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Considerable progress has been made in the knowledge of the epidemiology of gestational trophoblastic disease (GTD) in the last few years. There are two main and widely known points related to this disease: its geographical distribution and the different frequency in the various classes of age. GTD is more frequent in South-East Asia, India and Africa, and is rare in European and North American populations. For example, in the United States, the frequency of GTD was 108 per 100,000 pregnancies in the 1970's. In Europe, particularly in Italy, frequencies are lower. In northern Italy, the frequency of hydatidiform mole, in the period 1979-1982, was equal to 62 per 100,000 pregnancies, but in Indonesia and in China, the reported rates were 993 and 667 per 100,000 pregnancies respectively. GTD disease is more frequent in the extreme classes of age (under 20 and over 40 years) and the risk may be more than 100 times greater over 50 years. Besides these risk factors, the possible role of both genetic (familiarity, blood groups) and environmental factors (diet, cigarette smoking, etc.) has been investigated on the onset of GTD. This paper reviews the epidemiologic knowledge on GTD.