Malignant trophoblastic cells from a case of choriocarcinoma were cytogenetically investigated by direct analysis of fresh tissue from the tumor. To our knowledge, previous cytogenetic studies have been performed only on established cell lines. In this study, 54 metaphases were observed, of which 41 were fully karyotyped. Three chromosomally abnormal lines were identified. In all of them, trisomy 3 and 10, a supernumerary isochromosome 1q,i(1)(q10), an i(8)(q10) replacing one chromosome 8, and a marker chromosome were observed. In addition, involvement of chromosome 12 was observed in two of the three lines, trisomic in one and an i(12)(q10) in the other.