Because multiple risk factors in one patient may increase the clinical expression of thrombophilia, we assessed the presence in protein C- deficient patients of the factor V Arg 506 Gln mutation responsible for activated protein C resistance. Using a strategy allowing rapid screening of factor V exon 10, we studied 113 patients with protein C deficiency and 104 healthy volunteers. We detected the Arg 506 Gln mutation in 15 patients (14%) and in one healthy subject (1%). We identified a previously unpublished sequence variation leading to an Arg 485 Lys substitution in three normal subjects and seven protein C- deficient patients. A significant difference in the allelic frequency of the Arg 506 Gln factor V mutation was found between protein C- deficient patients heterozygous for an identified protein C mutation (n = 84; allelic frequency, 4.8%) and protein C-deficient patients with no identified mutation in the protein C gene coding regions (n = 25; allelic frequency, 14%). The results demonstrate that a significant subset of thrombophilic patients has multiple genetic risk factors although additional secondary genetic risk factors remain to be identified for the majority of symptomatic protein C-deficient patients.