A catheter for collecting fluid such as blood from a vessel such as a coronary sinus of a patient has an elongated, flexible tubular portion and an annular sealing member spaced from a distal end of the tubular member. A proximal end of the tubular member is adapted to be connected to a source of suction for applying suction to a collection lumen of the tubular member. The distal end is placed in a blood vessel with the sealing member spaced from opposing surface of the vessel in an absence of a suction applied to the collection lumen. Suction is applied to the collection lumen in an amount sufficient to draw blood from the vessel through the fluid inlet and into the collection lumen and further sufficient to urge the opposing surface of the vessel to migrate into sealing engagement with the sealing member.