Mercury strain gauge plethysmography with programme and ECG synchronous venous occlusion was tested in 14 normal subjects (28 lower limbs). The results obtained in the case of recent deep venous thrombosis of the lower limbs were compared with those of phlebography ni 39 patients: 26 proximal thromboses and 15 calf thromboses. Interpretation of the results must take into account the strict conditions under which the plethysmography was carried out: position of the subject and especially of the lower limbs and the technical characteristics of the apparatus used. Specificity was 96.4% for the group of normal subjects. Sensitivity calculated for all the cases of venous thrombosis studied was 97.5% whilst it was 100% for proximal thromboses (femoral and iliac).