It has recently been claimed (Ferrante et al., 1995. HTLV tax-rex DNA and antibodies in idiopathic amyotrophic lateral sclerosis. J. Neurol. Sci. 129 (Suppl.) 140-144) that human T-lymphotropic virus (HTLV) tax-rex sequences are detectable in the peripheral blood mononuclear cells (PBMCs) of 40% of patients with motor neurone disease (MND). In an attempt to confirm this we employed a highly sensitive 'nested' polymerase chain reaction (PCR) assay, capable of detecting single molecules of HTLV proviral DNA, to look for tax-rex sequences in the PBMCs of 43 patients with MND. We were unable to detect the presence of HTLV tax-rex in any of 43 MND patients tested, using three different PCR primer sets under both high and low stringency conditions. Using the same DNA samples we were able to detect the presence of the single-copy pyruvate dehydrogenase gene, thus demonstrating that the extracted DNA was indeed amplifiable by PCR. To further exclude the possibility that the extracted DNA samples contained unrecognised inhibitory factors we conducted spiking experiments with trace amounts (approximately 10 copies) of HTLV proviral DNA. Spiked samples yielded PCR products of the expected size. We are therefore unable to confirm the presence of HTLV tax-rex sequences in this disease.