OBJECTIVE To determine whether superficial thrombophlebitis (STP) can extend into the deep venous system (DVS) and whether this may result in pulmonary embolization. METHODS All venous duplex ultrasound examinations performed in our vascular laboratory to rule out deep venous thrombosis from June 1, 1994, to June 24, 1996, were reviewed. RESULTS Of 8313 limbs studied by duplex scanning in 6148 patients, 1756 limbs (21.1%) had a positive result for deep venous thrombosis. STP was demonstrated in 232 limbs (213 patients), of which 20 (8.6%) extended into the DVS. Fourteen (70%) were noted on the initial scan, and six (30%) were detected on serial follow-up scans. Eighteen (90%) originated in the proximal greater saphenous vein and extended across the saphenofemoral junction into the common femoral vein. Nine demonstrated "free-floating" thrombus with a "tongue" extending into the common femoral vein while still attached proximally to the greater saphenous vein. Extension of thrombus from the lesser saphenous vein into the popliteal vein was noted in two cases (10%). One pulmonary embolization was directly observed to occur in real time during scanning. No pulmonary embolization was seen when STP did not involve the DVS. CONCLUSIONS STP can extend into the DVS. In this series STP of the proximal greater saphenous vein extended into the common femoral vein in 8.6% of the cases, of which 10% embolized to the lungs. When the DVS is involved, standard deep venous thrombosis treatment (heparin, warfarin, bed rest) should be instituted. We recommend duplex imaging for STP involving the greater saphenous vein in the thigh to rule out occult deep venous thrombosis. STP is not always benign and self-limiting as previously described.