Mucinous tubular and spindle cell carcinoma of the kidney is a new diagnostic entity. We present the pathologic and genomic characteristics of three such low-malignant tumors. Two of the tumors were found in women aged 19 and 52 years, the third tumor was found in an 80-year-old man, and the tumor stages were pT2N0MX, pT2NXMX, and pT1NXMX, respectively. Findings by immunohistochemistry were similar but not identical for the three cases; markers for both proximal and distal parts of the nephron were expressed in each tumor, a finding that is in agreement with data from previous studies. The Ki-67-labeling index was below 5 in all three cases. Two of the tumors were predominantly hypodiploid (DNA-indexes 0.77 and 0.80), whereas the third tumor was hypertriploid (1.57) as measured by DNA-image cytometry. From the latter tumor live cells were available making it possible to establish its karyotype: 62-70,XXX,+del(X)(q11),-1,+2,+4,-5,-6,+7,-8,-9,-10,-11,+12,-13,-14,-15,+16,+17,+18,-19,+20,+21,-22[cp15]. Interphase fluorescence in situ hybridization analyses with centromere-specific probes for chromosomes 1, 3, 4, 6, 7, 9, 10, 17, 18, 20, and X showed that the two hypodiploid tumors had disomic and monosomic chromosome populations, whereas the karyotyped, near-triploid tumor was dominated by trisomic chromosome populations. Comparative genomic hybridization analysis was normal for the karyotyped tumor but abnormal for the two others. We conclude that multiple numerical chromosome aberrations may be a feature of mucinous tubular and spindle cell carcinomas of the kidney, but beyond that no clear-cut karyotypic aberration pattern is so far discernible.