In this study we investigated a group of patients in whom a resistance to APC (activated protein C) was found but no Leiden mutation existed in the presence of missense mutations in the first 1200 bp of the Exon 13 (B-domain) in the factor V (FV) gene. The determination of the APC response was performed using the Immunochrom(R) APC response Test Kit. The mutations were determined by temperature gradient gel electrophoresis and DNA sequencing. In the APC-resistant patients without the FV Leiden, we found 4 silent mutations (2298C>T, 2325T>C, 2379A>G, 2391A>G) and 4 missense mutations (2540A>C, 2663A>G, 2684A>G, 2863A>G), which code for the amino acids N789T (GenBank Accession # AF119360), K830R, H837R, and K897E. In all of the patients and controls, the polymorphisms at nucleotide positions 2391, 2663, 2684, and 2863 appeared to be associated. In the major allele all bases are A (A allele) and in the minor allele are G (G allele). A significantly lower G allele frequency was observable in the patient group than in the control group (0.14 vs. 0.31; p<0.05). The frequency of the 2540C allele, which is associated with the 2379G and the 4070G allele (non-Leiden!), did not differ significantly between the patient and the control groups. We suggest that the G allele, which is not associated with the FV Leiden mutation, as well as the [2379G; 2540C; 4070G] allele have no influence on the APC cofactor function itself, or only subtly as determined in the test systems used.