Three pediatric germ cell tumors (GCT) were cytogenetically analyzed. A mediastinal mature teratoma in a 15-year-old girl had a balanced t(8;22)(p21;q12) as the sole clonal aberration, an intrathoracic immature teratoma in a 2-year-old girl had the complex karyotype 46,XX,der(6) ins(6;2)(q15;q11q23),de1(8)(q22),-1O,der(12)t(10;12),(q22;q22- 23),der(16)t(1;16)(q12;q11),+mar and a congenital presacral endodermal sinus tumor was characterized by the karyotype 47,XY,add(11)(p15),der(13)t(1;13)(q21;p13),add(14)(p13),d e1(15)(q24),+der(?)t(?;11)(?;q13). The present three tumors had no chromosome aberration in common, nor has any specific change been detected in the 13 previously reported cytogenetically aberrant pediatric GCT. The karyotypic picture comes across as far more heterogeneous than that of GCT of adults. Whereas gain of 12p material, in the vast majority through i(12)(p10) formation, dominates in the adult setting, the most common cytogenetic abnormalities in pediatric GCT seem to be unbalanced recombinations leading to gain of 1q. Other recurrent changes include, in decreasing order of frequency, numerical and structural aberrations leading to gain of 8q and 12p, loss of distal 1p, +3, loss of 7q22-32, -10, -13 and -18.