We present a case report of a young man with spontaneous deep venous thrombosis. He tested positive for heterozygote factor VLeiden mutation and was treated with warfarin. However, he turned out to be resistant to warfarin, and another venous thrombosis occurred during the insufficient treatment. The antithrombotic treatment was then successfully replaced by phenprocoumon. This case report emphasizes the importance of critically evaluating the efficacy of a treatment and substitute if proven insufficient.