Recent association studies investigating polymorphisms in the alpha2-macroglobulin (A2M) gene provided evidence for an involvement of this protease inhibitor in the pathogenesis of Alzheimer's disease (AD). The partially overlapping pathology between AD and Parkinson's disease (PD) led us to investigate the role of A2M in PD. We performed association studies in a large sample of 328 German PD patients and 322 closely matched healthy controls. Analyzing the Val1000Ile polymorphism and a pentanucleotide deletion in the 5' splice site of exon 18 of the A2M gene we found an excess of homozygosity for the A2M deletion in early-onset PD (EOPD) patients (age at onset < 50 years) compared to late-onset PD (LOPD) patients (age at onset > 50 years; p = 0.008, p(p)c = 0.064, chi2 = 7.017). Therefore our data might indicate an age at onset modulating effect of the homozygous A2M deletion in PD.