Dutch researchers reported that people who develop blood clots in the veins, will double their siblings or childrens risk to develop the condition than those without a family history. This kind of clot known as venous thrombosis can be potentially dangerous, because it can break off and travel to the lungs. The researchers compared 1,605 venous thrombosis patients with a control group of 2,159 people who never had the condition. The study showed 505 (31.5 percent) of the venous thrombosis patients had at least one first-degree relative with a history of the condition, compared with 373 (17.3 percent) of those in the control group. The risk was even more if the relative developed blood clots at a young age and as much as four times higher if more than one relative had venous thrombosis. The researchers said that there wasn't a strong link between family history and known genetic risk factors, which suggests that there may be unknown genetic risk factors or that venous thrombosis may affect members of the family due to household factors. They also suggested that family history may be more useful for risk assessment than laboratory tests that identify genetic or physiological risk factors.