The incidence of thrombosis during one year in a defined population of 263 144 city inhabitants has been studied with a new method based upon acute phlebography of all patients with the slightest symptoms of the disease. The roentgenographic pattern of the acute thrombosis has been studied and the size of the individual thrombus has been compared with the duration of the symptoms. The incidence of thrombosis was found to be 0.9 per mille per year. No correlation was found between the size of the thrombus and the duration of the symptoms. The thrombus had its origin in the calf veins in 89% of the patients and its origin in a vein at a higher level in 10% of the patients. In patients with a thrombus confined to the calf veins the contrast medium always entered the deep crural veins. When the calf vein thrombosis had an extension to the popliteal vein or still higher up the contrast medium bypassed the deep crural veins in more than 75% of the patients.