Measurement of fibrin degradation products (FDP) was evaluated in 152 consecutive patients referred because diagnosis of deep vein thrombosis (DVT) was suspected. All patients underwent impedance plethysmography (IPG), and venograms were obtained in 59. Sensitivity and specificity of FDP measurement, using venography as the standard, was 88% and 66%, respectively. Sensitivity and specificity of IPG was 77% and 79%, respectively. Specificity of the two tests combined, when both yielded abnormal results, was 93%. Patients were classified as (1) normal, (2) abnormal, or (3) equivocal on the basis of IPG. A significant difference in mean FDP concentration was found between groups 1 and 2 and groups 2 and 3. An overall correlation was found between degree of abnormality on IPG and FDP concentration. A quantitative determination of FDP is a practical and reliable screening test for DVT and, when used in conjunction with IPG, improves both sensitivity and specificity of the latter.