Coagulation factor V (FV) plays an important role in maintaining the hemostatic balance in both the formation of thrombin in the procoagulant pathway as well as in the protein C anticoagulant pathway. FV deficiency is a rare bleeding disorder with variable phenotypic expression. Little is known about the molecular basis underlying this disease. This study identified 5 novel mutations associated with FV deficiency in 3 patients with severe FV deficiency but different clinical expression and 2 unaffected carriers. Four mutations led to a premature termination codon either by a nonsense mutation (single-letter amino acid codes): A1102T, K310Term. (FV Amersfoort) and C2491T, Q773Term. (FV Casablanca) or a frameshift: an 8-base pair deletion between nucleotides 1130 and 1139 (FV Seoul(1)) and a 1-base pair deletion between nucleotides 4291 and 4294 (FV Utrecht). One mutation was a novel missense mutation: T1927C, C585R (FV Nijkerk), resulting in the absence of mutant protein despite normal transcription to RNA. Most likely, an arginine at this position disrupts the hydrophobic interior of the FV A2 domain. The sixth detected mutation was a previously reported missense mutation: A5279G, Y1702C (FV Seoul(2)). In all cases, the presence of the mutation was associated with type I FV deficiency. Identifying the molecular basis of mutations underlying this rare coagulation disorder will help to obtain more insight into the mechanisms involved in the variable clinical phenotype of patients with FV deficiency.