The majority of solitary fibrous tumors (SFTs) of the pleura are benign, but 10-30% locally recur or metastasize. Pathogenic factors relevant to the determinism of their biological properties are largely unknown. Cytogenetic data on SFTs of the pleura are sparse. We report herein a case of a malignant SFT of the pleura where successful karyotyping was obtained from the primary and recurrent tumors. The initial karyotype showed two abnormal clones: 48, XY; +8; +8; del(9)(q22; q32) [19] and 46, XY, t(1;16)(q25;p12) [7]. Culture of the recurrent tumor yielded one clone identical to the dominant clone of the initial karyotype. Demonstration of a recurrent abnormal karyotype largely supports its relevance to the malignant clone and suggests a role of supernumerary chromosome(s) 8 in the determinism of malignant behavior in SFT.