Mutations in nucleophosmin (NPM1) exon 12 are thought to be the most common genetic event in acute myelogenous leukemia (AML) and to confer favorable clinical prognoses. In this report, we describe a simple molecular test for the detection of NPM1 exon 12 mutations in patients with AML using polymerase chain reaction amplification of genomic DNA followed by the analysis of amplification products by capillary electrophoresis. Mutations were reproducibly detected when present in at least 5% of cells, and all NPM1 exon 12 mutations reported to date in AML could be identified using this method. This method was successfully employed using paraffin-extracted DNA, allowing for the examination of archived clinical specimens, and the assay was validated by the direct sequencing of 33 patient samples. This sensitive test is straightforward to perform and provides important information that can influence both the clinical management and treatment options for many patients with AML.