To investigate the aetiology of hydatidiform mole, laminin-1 expression was determined in human villous tissues obtained from normal pregnancies (n = 17) and complete hydatidiform moles (n = 10). Indirect immunofluorescent staining was performed to detect laminin-l, and Northern blot analysis was performed to assess expression of laminin mRNA. Serum concentrations of laminin P-1 were also measured. Immunohistochemical analysis revealed stronger staining of the trophoblastic basement membrane in hydatidiform mole than in normal pregnancy. Northern blot analysis revealed that villous expression of laminin mRNA was significantly increased in hydatidiform mole compared with normal pregnancy (P < 0.05), The serum laminin P-1 concentration was also significantly higher in those patients with hydatidiform mole than in normal pregnancy (P < 0.05). These results suggest that laminin-1 might play an important role in determining the pathophysiology and structure of hydatidiform mole. In addition, measurement of serum laminin P-1 in combination with human chorionic gonadotrophin might be useful in the diagnosis of hydatidiform mole, to evaluate the prognosis, and to detect the presence of metastatic or recurrent disease.