The purpose of this study was to analyze the various factors that influence the anatomical site of occurrence of DVT and to determine if the clinical course differed in patients afflicted with DVT at different anatomical sites in the lower extremity. Forty four of 92 patients undergoing venography during a 4-1/2 year period had positive venograms for DVT. Patients were grouped into one of three categories: iliofemoral thrombosis (IFT) n = 9, superficial femoral vein thrombosis with or without distal thrombosis (SFV) n = 21, and popliteal/calf thrombosis (clot limited to below the knee) (PCT) n = 14. Patients in the IFT group had a significantly prolonged hospital stay (p less than .05) and a significantly lower mean weight (129 lbs) when compared to the PCT group (173 lbs) (p less than .05). Pain was present equally among the three groups. Swelling was much more common in the SFV group, whereas tenderness was most frequent in the PCT group. Of those patients with swelling, 70% were in the SFV group and of those patients with tenderness, 60% were in the PCT group. DVT as the primary diagnosis was seen in 39% of cases of which half had disease limited to the PCT region. Post-op DVT occurred equally among the groups. DVT occurred much more frequently in the PCT region after myocardial infarctions and after orthopedic procedures, whereas in patients with malignancies, the most common site was the SFV region. Pulmonary embolism developed in 11% of patients and occurred in the IFT and SFV groups only. No patient with DVT of the calf/popliteal developed a pulmonary embolism.(ABSTRACT TRUNCATED AT 250 WORDS)