Abstract: A surgical instrument (10) for removing a sample (50, 82) from a body of tissue (80) comprises a support structure (30) and a knife (32) arranged to cut into the body of tissue (80). The knife (32) has at least a first blade (36a), a second blade (36b) and a third blade (36c), and each blade has two cutting edges (42a, 44a; 42b, 44b; 42c, 44c) defining an apex. The knife (32) is movable between a first configuration in which the blades (36a, 36b, 36c) are substantially aligned along the longitudinal axis (X) of the support structure (30), and a second configuration in which the blades (36a, 36b, 36c) define the walls of a pyramidal enclosure for containing the sample (50, 82). In one embodiment, the instrument (10) further comprises a biasing member (52a, 52b, 52c) arranged to urge each blade (36a, 36b, 36c) towards the central axis (X) of the support structure (30).