The 19-bp insertion/deletion polymorphism in the 5' flank of the dopamine beta-hydroxylase (DBH) gene has been associated with psychiatric disorders. We have developed a simple, reliable and inexpensive closed-tube assay for genotyping of this polymorphism based upon T(m) determination of amplified DNA fragments. Mistyping of heterozygote samples due to preferential allele amplification was prevented by use of an optimized concentration of Mg(2+), addition of dimethyl sulfoxide and annealing/extension at an appropriate temperature. Comparison of results achieved by the closed-tube assay and a conventional approach based upon agarose gel electrophoresis of amplified fragments revealed complete concordance between the two procedures. The insights obtained in this study may be utilized to develop assays based upon dissociation analysis of PCR products for genotyping of other insertion/deletion polymorphisms.