This study highlights the importance of error analysis in providing a comprehensive profile of an individual's grammatical ability with regard to relative clause (RC) constructions. The aim was to identify error patterns in the production of RCs by English-speaking, school-aged children with specific language impairment (SLI) and to relate them to their level of competence with these structures. Children with SLI (mean age = 6; 10, n = 32) and two control groups - a typically developing group matched for age (mean age = 6; 11, n = 32) and a younger typically developing group (mean age = 4; 9, n = 20), repeated sentences containing RCs that represented a range of syntactic roles. Data are presented on three distinct error patterns - the provision of simple sentences, obligatory relativizer omission and RC conversions. Each is related to the level of competence on RCs that each child has achieved.