We sought to determine how rates of sonographically detected deep venous thrombosis correlate with presenting symptoms in cancer patients. We performed venous sonography in 588 cancer patients with clinically suspected lower extremity deep venous thrombosis. Results were correlated with clinical findings. Deep venous thrombosis was diagnosed in 32% of patients with unilateral lower extremity symptoms and in 17% of patients with bilateral symptoms. Patients with unilateral symptoms of pain and swelling, swelling alone, or pain alone had significantly different rates of deep venous thrombosis (47%, 31%, and 16%, respectively). In patients with bilateral leg symptoms, deep venous thrombosis was significantly more likely when symptoms were not bilaterally symmetric.