Short-term cultures from a clear cell hidradenoma, a benign skin tumor for which no chromosome data exist, were cytogenetically analyzed. A total of eight unrelated aberrant clones were identified. The karyotypic profiles of two separately processed parts of the sample--a tumor nodule and seemingly normal adjacent dermal tissue--were different. Characteristic for the tumor nodule was a single abnormal clonal population consisting of three subclones: 46,XY,der(2)inv(2)(p13q23)t(2;9)(p13;q22), der(9)t(2;9)(q23;q22),t(11;19)(q21;p13),t(12;19)(q24;p13)/46,idem, inv(1)(p32q44)/92,idemx2. The adjacent tissue contained, in addition to the clone found in the tumor nodule, a spectrum of unrelated clones, the largest of which also showed clonal evolution: 45-47,XY,t(3;6)(p25;p25),t(12;17)(q15;q12),-17,+r(17)x2 [cp]/45-47,idem,inv(5)(p15q22)/90-94,idemx2. The remaining six clones found in this part were small and had simpler numerical or structural aberrations. The multiclonal pattern observed in this hidradenoma seems to reflect both cytogenetic convergence and divergence during neoplastic progression. The presence of unrelated clones may be an indication that the tumor was of multicellular origin.