Pigmented villonodular synovitis (PVNS) is a proliferative lesion of disputed genesis. Recently, we reported trisomy 7 in short-term cultures of 1 PVNS. In the present report, we describe another specimen of PVNS in which 9 of 26 (35 percent) metaphase cells demonstrated trisomy 7 when analyzed after 3-15 days of tissue culture. In situ hybridization analysis, with a biotinylated probe to chromosome 7 alpha-satellite DNA, revealed trisomy 7 in 53 of 200 uncultured cells from this PVNS sample. Our findings indicate that trisomy 7 is a nonrandom aberration that arises in vivo in PVNS.