We have developed a technique to diagnose the alpha- and beta-thalassemia (thal) syndromes using the polymerase chain reaction to amplify cDNA copies of circulating erythroid cell messenger RNA (mRNA) so as to quantitate the relative amounts of alpha-, beta-, and gamma-globin mRNA contained therein. Quantitation, performed by scintillation counting of 32P-dCTP incorporated into specific globin cDNA bands, showed ratios of alpha/beta-globin mRNA greater than 10-fold and greater than fivefold increased in patients with beta 0- and beta (+)-thal, respectively, as well as a relative increase in gamma-globin mRNA levels. Conversely, patients with alpha-thalassemia showed a decreased ratio of alpha/beta-globin mRNA proportional to the number of alpha-globin genes deleted. This methodology of ascertaining ratios of globin mRNA species provides a new, simplified approach toward the diagnosis of thalassemia syndromes, and may be of value in other studies of globin gene expression at the transcription level.