An orthopaedic surgical instrument for positioning an acetabular prosthetic component in a patient's surgically-prepared acetabulum has a base (48) configured to engage the patient's pelvis, a first linkage (50) pivotally coupled to the base, a locking mechanism operable to lock the first linkage in position relative to the base, and a second linkage (56) removably coupled to the first linkage. The second linkage has an alignment axis (54) corresponding to a desired abduction angle and a desired anteversion angle of the acetabular axis of the acetabular prosthetic component.