The purpose of this study was to define the incidence of and outcomes associated with isolated acute calf vein thrombosis (CVT). From 11/95 through 6/97, 3096 patients underwent lower extremity venous duplex testing in a hospital-based vascular laboratory in which bilateral tibial and peroneal vein imaging were standard components of the venous duplex examination. CVT was present in 118 patients (3.8%), and 339 patients (10.9%) had acute proximal deep venous thrombosis (PDVT). Patients with CVT were 56.4+/-17.2 years of age (range, 18-92). Approximately 25 per cent with CVT had cancer (n = 30). Of the 18 patients with CVT who underwent ventilation-perfusion (V/Q) lung scanning, 56 per cent (n = 10) had high-probability scans. Venous duplex reports for those with CVT recommended follow-up venous duplex examination, which was done in 60 per cent (n = 71) of patients. Of the 71 patients with CVT who underwent follow-up testing, 15.5 per cent (n = 11) progressed to PDVT. The incidence of progression to deep venous thrombosis was 25 per cent (9 of 36) in those receiving anticoagulants at the time of initial venous duplex examination versus 5.7 per cent (2 of 35) in those not receiving anticoagulants (P = 0.046). With progression to PDVT, patients were more likely to have cancer (35% versus 7.8%; P = 0.009), more likely to have high-probability V/Q scans (36% versus 6.7%; P = 0.017), and more likely to die (27% versus 1.7%; P = 0.011) during follow-up. CVT was less common than proximal deep vein thrombosis and was also associated with pulmonary embolism. Progression of CVT was an adverse clinical event associated with greater chance of pulmonary embolism and death.