A retroperitoneal neurofibrosarcoma infiltrating the spine of a 2-year-old boy was investigated cytogenetically three times over a 5-month period. The first sample, from a diagnostic fine-needle aspiration biopsy, had a supernumerary i(1)(q10) as the sole clonal aberration; two cells showed monosomy 18 in addition to the isochromosome. The second sample, obtained at tumor resection 3 weeks later, had the karyotype 47,XY, +i(1)(q10), -18, +21/45,XY, -18. After 5 months, a large local recurrence was resected. The chromosome analysis showed further clonal evolution: 45,XY, +1,der (1;11)dic(1;11)(q44;q13)i(1)(q10), inv(6)(p21q12), -17. The findings indicate that formation of i(1)(q10) and loss of chromosome 18 may be early genetic events in neurofibrosarcoma development.