We describe the karyotypes of nine Wilms tumors (WT). Four tumors were initially karyotyped from diagnostic needle core biopsies, 3 after postchemotherapy tumor resection and the remainder from xenografts grown in nude mice. The 9 nephroblastomas were composed of 7 with favorable histology (intermediate-grade malignancy) and 2 with unfavorable histology (anaplastic or high-grade malignancy). The 7 tumors with favorable histology had karyotypes typical of WT, with the previously described nonrandom abnormalities +1q, +6, +7, +8, +12, +13, +18 and structural abnormalities of 1p and 16q present in at least 1 case. The most common abnormalities were trisomy 18 (4 cases) and +1q (3 cases). The 2 tumors with unfavorable histology both had complex karyotypes atypical for WT. We suggest that cytogenetics can act as a marker when histologic grade is in doubt. Karyotypic analysis from needle core biopsies was attempted in 6 samples, including 1 from a nephrogenic rest (NR) of the nonaffected kidney and provided a result on 5 occasions. The NR were present in the sole case with a constitutional abnormality, a mosaic partial duplication of 8q. However, both the tumor and the NR were apparently derived from the normal cell line. Here we demonstrate that a cytogenetic result can be routinely obtained from needle core biopsies and will thus facilitate true diagnostic tumor karyotypes in both WT and other tumors.