OBJECTIVE: Superficial thrombophlebitis (ST) is a frequent and potentially serious disease if complicated with venous thromboembolism (VTE). Data on risk factors and incidence rates for ST are scarce. It is also unknown whether ST is a risk factor for recurrence of VTE. METHODS: After discontinuation of secondary thromboprophylaxis for a first spontaneous VTE, we prospectively observed 615 patients on average for 30 +/- 26 months. Patients with natural coagulation inhibitor deficiency, lupus anticoagulant, or cancer, who were pregnant, or were receiving long-term antithrombotic therapy were excluded. The study outcomes were occurrence of symptomatic ST or objectively documented recurrent symptomatic VTE. RESULTS: ST developed in 45 patients (7.3%) with a first VTE. High factor VIII concentration emerged as an independent risk factor for ST (relative risk [RR], 2.0; 95% confidence interval [CI], 1.0-5.2), compared with lower levels after adjustment for age and sex; factor V Leiden and prothrombin G20210A concentration; hyperhomocysteinemia; high body mass index; and duration of oral anticoagulation therapy. VTE recurred in 12 (27%) of 45 patients with ST and in 67 (12%) of 570 patients without ST. In patients with VTE, subsequent ST emerged as an independent risk factor for recurrent VTE. Patients with ST had twofold higher RR (2.1; 95% CI, 1.0-4.2) for recurrence than did patients without ST after adjustment for putative confounding variables. CONCLUSION: Patients with a first spontaneous VTE and subsequent ST are at increased risk for recurrent VTE. High factor VIII concentration is an independent risk factor for ST.