Human neurocysticercosis (NC) is caused by Taenia solium larvae lodged in the central nervous system. This disease is usually diagnosed by radiology but the results are not always clear-cut and so immunological assays are often also used. A semi-nested PCR, based on the non-coding HDP2 sequence of T. saginata, has now been developed for detecting DNA from T. solium cysticerci and confirming NC. This PCR, which amplifies a 171-bp T. solium product, allowed the specific detection of just 174 attograms of T. solium DNA. The efficacy of the PCR was tested using cerebrospinal fluid (CSF) from neurological patients, including 46 confirmed Mexican cases of NC and 32 patients from non-endemic Spain. Eighteen of the confirmed cases [including 10 (71%) of the 14 with vesicular extraparenchymal cysticerci and four (17%) of the 24 with damaged cysticerci] and two (33%) of the six patients with 'uncertain' diagnosis (in whom a diagnosis of NC could not be established by radiological and immunological studies) were found PCR-positive. The 36 patients known to have neurological problems other than NC were found PCR-negative. The HDP2 PCR offers a new tool in the diagnosis of NC and in exploring the pathogenesis of this serious disease.