We have, during a 12-month period, evaluated the adjuvant effect of combining G-band karyotyping and multi-color spectral karyotyping (SKY) in acute leukemia patients. Forty-four cases were evaluated; fewer cases than those routinely analyzed by G-band cytogenetics had mitoses left for SKY analysis. Of the 44 patients, 35 were acute myeloid leukemia (AML) and 9 acute lymphatic leukemia (ALL) cases. Twenty-seven of 35 AML and 7 of 9 ALL patients had an abnormal G-band karyotype. Thirteen of these 34 abnormal cases had a simple clonal chromosome aberration, and the remaining 21 cases had a complex karyotype. The SKY confirmed the simple karyotype in 11 and in 7 with a complex karyotype. In 13 of the cases with a complex karyotype, ambiguous structural aberrations were classified, in 6 of these, SKY disclosed cryptic translocations. Thus, SKY either extended or confirmed G-band karyotypes in 31 of 34 analyzed abnormal cases. Cases where SKY did not reveal the abnormal clone showed only few abnormal mitoses by G-banding (2/23, 2/25, and 4/27). Additional or confirmatory information was therefore obtained in 91% of analyzed cases, and SKY proved to be a valuable additional tool for hematologic cytogenetics.