Purpose: Overall prevalence of pulmonary embolism (PE) in patients with deep venous thrombosis (DVT) isolated to calf veins is low. However, the prevalence of PE in the subgroup of patients with respiratory symptoms and isolated calf vein thrombosis (CVT) is unknown. Such information is important in determining whether patients with CVT only and respiratory symptoms should undergo evaluation for PE. The purpose of this study was to determine the prevalence of PE in patients with respiratory symptoms and isolated CVT. Methods: From 1992 through 1994, all patients assessed by duplex scanning for lower extremity DVT were reviewed, and those found to have isolated CVT and lower extremity or respiratory symptoms were identified. Patients who had respiratory symptoms or later developed respiratory symptoms in addition to lower extremity symptoms underwent pulmonary angiography or ventilation/perfusion (V/Q) scanning. Positive results on pulmonary arteriograms or 'high probability' V/Q scans were considered diagnostic of PE. Results: There were 105 patients with isolated CVT and symptoms. Twenty- six patients had respiratory symptoms; nine (35%) had PE and two died. Seventy-nine patients had only lower extremity complaints; five later developed respiratory symptoms. All five had PE and none had progression of CVT on repeat duplex scanning. Neither age, gender, prior DVT/PE, obesity, pregnancy, medication, known malignancy, smoking, recent surgery, or trauma predicted PE. Conclusions: Patients with respiratory symptoms and duplex diagnosed isolated CVT have a high prevalence of PE and require pulmonary angiographic or V/Q scanning to rule out PE.