Impedance rheography in conjunction with a respiratory maneuver has proven to be useful in the detection of deep venous thrombosis of the legs. In a study of 25 normal volunteers, there were no changes in base-line impedance of less than 0.19% following deep inspiration. Two hundred fortyfive patients were studied; 45 patients had responses at or below 0.19%, and in 40 cases, deep venous thrombosis was proven or strongly suspected. In a prospective study of 64 patients, deep venous thrombosis was diagnosed six times and confirmed by venography in each case. However, four patients exhibited a false-positive result which appears to be a significant limitation of the method. Preliminary studies have shown that variation in the respiratory maneuver may eliminate the false-positive responses.