Historical and clinical aspects of chromosome analysis by flow cytometric methods are reviewed. A new method of preparing small samples (1.0 mL of blood) of peripheral lymphocytes for flow karyotype analysis using phytohemagglutinin and interleukin-2 is presented in detail. Figures of flow karyotypes, partial banded karyotypes, and idiograms of patients with inv(8), rec dup(8), rob t(14;21), and t(1;22) are presented, as well as examples of univariate and bivariate flow histograms from other researchers' published works. The clinical utility of these techniques is explored, with specific reference to recent work in chromosome polymorphisms and cultured amniocyte lines. We conclude that, although flow karyotyping is not a replacement for classical banded chromosome analysis, used as an adjunct, this new technique has some clinical usefulness relating to its capability to resolve small differences in chromosomal DNA content.