We report the successful application of interphase fluorescent in situ hybridization (FISH) using extracted nuclei from snap frozen buffy coat stored for the purpose of molecular analysis. Included in this study were 30 frozen buffy coat specimens, ranging from <1 day to 3 yr old. Thawing of the buffy coat at 37 degrees C was followed by lysing erythrocytes with lysing solution (LYSE S III DIFF, Coulter Corp.). Cells were fixed and slides were prepared according to manufacturer's FISH protocol. All specimens from the 30 frozen-thawed buffy coats were suitable for FISH evaluation, and signal intensities were not significantly related to the age of specimens. In conclusion, interphase FISH is feasible using frozen-thawed buffy coat; the technique will be useful for retrospective molecular cytogenetic analysis of hematologic malignancies.