Reliability of goniometric measurements of ranges of motion in the right hip of four children with mild or moderate spastic diplegia was studied by comparing results when physical therapists used specific and non-specific measurement instructions. Measurements of hip extension, abduction and external rotation were made. The use of specific measurement instructions improved inter-rater reliability only in the case of external rotation. Inter-session reliability was not improved by the use of specific measurement instructions. It is concluded that goniometric measurements have such a low level of reliability that they can only be used to assist clinical judgment, and that they are not sufficiently reliable to be used in studied of cerebral palsy.