Alveolar echinococcosis is a life-threatening parasitosis occurring in countries in the Northern hemisphere. The diagnosis of an Echinococcus multilocularis infection is routinely performed by radiological techniques and by the detection of specific antibodies in the sera of infected patients. However, because serodiagnosis fails in 5%-10% and radiological techniques are difficult to interpret in some cases, we developed a polymerase chain reaction for the detection of echinococcal-specific messenger RNA from fine-needle biopsy specimens. This technique was applied to the diagnosis of alveolar echinococcosis in a 20-year-old seronegative woman. Detection of messenger RNA not only allowed the diagnosis of echinococcal disease but also proved to be a reliable measure for determining the efficacy of an antiparasitic therapy.