Fifteen patients presented during a four-year period with a pseudothrombophlebitis syndrome, subsequently confirmed arthrographically as being due to popliteal cyst rupture. Initially the majority of patients (73%) were erroneously diagnosed as having calf vein thrombosis, and were anticoagulated for periods of three to ten days. Clinical differentiation from venous thrombosis was usually impossible. Contrast arthrography was the definitive diagnostic investigation, revealing popliteal cysts in all patients, and an active synovial leak into the calf in 13 cases (87%). The mean delay in performing arthrography was 5.3 days, the procedure usually being performed after normal contrast venography. Doppler and isotope venography yielded misleading or equivocal results (42%) delaying diagnosis, and prolonging periods of potentially dangerous anticoagulation. We suggest that the marked overlap in the emergency presentation of popliteal cyst rupture and calf vein thrombosis mandates the aggressive use of arthrography combined with venography in all patients presenting with a painful swollen leg.