Cytogenetic analysis of short-term cultured cells from an 11-cm adrenocortical carcinoma in a 3.5-year-old girl revealed the karyotype 46,XX,inv(9)(p11q12)c/[2]/56-57,XX,+2,+4,+5,+7,+8,inv(9)c,+10,+add (13)(p11), +14,+15,+19,+20,+20,+mar[cp19]. To our knowledge, this is the first description of an abnormal karyotype in a pediatric adrenocortical tumor. Inasmuch as the distinction between benign and malignant adrenocortical tumors is often difficult to make from clinical and histopathologic data alone, the present findings suggest that cytogenetic analysis may be a valuable adjunct in the differential diagnosis.