Abstract: In the field of echocardiography there is a need for improved guiding of elongate flexible medical instruments to shorten procedures and thereby reduce the exposure of an interventional cardiologist or cardiac surgeon, and an echocardiographer, to radioactive fluoroscopy. A guide unit (10; 70; 150), for an elongate flexible medical instrument (12) steerable by an operator handle (60), comprises a guide base (14) that is fixedly securable in an operative position to define a reference datum (16) relative to an orifice into which the flexible medical instrument (12) is, in-use, to be inserted. The guide base (14) supports a guide assembly (28) which is engageable, in-use, with the flexible medical instrument (12) to selectively inhibit lengthwise axial movement of the flexible medical instrument (12) relative to the guide base (14).