To assess whether trace elements are involved in Parkinson's disease (PD) we have conducted a prospective study where 19 trace elements have been determined in serum collected from 33 patients before they were diagnosed with the disease, and 99 controls. As a follow-up, serum from 19 of the same patients collected 4-12 years after they were diagnosed with PD has been analysed. In the prospective part of the study, the only significant difference was a slightly lower content of Hg in the patient group than in the controls. In the follow-up, significantly higher levels of Hg, Ni, and Y, and lower levels of Ca, Fe, Mg, Mn, Rb and Se were found in the serum samples collected after the patients were diagnosed with PD compared with pre-diagnostic levels.