Three cases in which venous obstruction in the thorax was demonstrated by visualization of collateral circulation during bolus-injection excretory urography are presented. Visualization of these collateral vessels is highly suggestive of venous obstruction even if this condition is clinically unsuspected. The diagnosis may be confirmed by radionuclide venography, which is easily performed and noninvasive, and allows determination of the level of obstruction, as well as the degree of collateral flow.