Certain non-invasive techniques like Doppler CW and strain gauge plethysmography are highly effective ways of evaluating the natural history of postphlebitic syndrome. 40 patients of both sexes, average age 51.5 +/- 6.9 with a history of deep venous thrombosis (28 documented phlebographically) were subjected to Doppler tests with assessment of venous blood pressure and bilateral strain gauge plethysmography. 34 of the 40 suspected PPS cases were confirmed, though not all cases were at the same stage. Persistent deep venous thrombosis was found in five of the extremities and the last one examined revealed a primary varicose syndrome. A comparison of the Doppler and phlebography results showed both to be highly sensitive techniques (100% accuracy). When the contralateral limbs were examined, the Doppler technique revealed 7 cases of PPS and 21 primary varices. In contrast strain gauge plethysmography identified all 28 cases of increased venous capacitance as primary varices, thus confirming the inability of this technique to distinguish between the various varicose conditions. Assuming the presence of a vascular diagnosis laboratory where both techniques are available, strain gauge plethysmography is recommended as the examination of choice. This technique is simple and fast to perform and can provide extensive information whether at rest (filling and emptying volumes and times; venous tone and distensibility, venous blood pressure at rest) or in movement (venous pressure when standing, muscular pumping index). Hence plethysmography can reveal any canalisation present even in the earliest stages though it cannot pinpoint the precise site of the deep obstruction. The longer, more complex Doppler CW procedure should be reserved for secondary investigations. This technique is preferable to plethysmography when a more accurate assessment of the degree, site and extension of the venous recanalisation is needed. Doppler CW also provides information on any valvar sequelae since it records the direction of the blood flow in the presence of a substitution syndrome (increased venous flow in the surface vessels). Finally if used in a rational manner the two techniques can be combined to eliminate contrast medium techniques, which would only be adopted as a preoperative measure.