Background: Children who meet language test criteria for specific language impairment (SLI) are not necessarily the same as those who are referred to a speech and language therapist. Aims: To consider how far this discrepancy reflects insensitivity of traditional language tests to clinically important features of language impairment. Methods & Procedures: A total of 245 twin children, 52 of whom had been referred to a speech and language therapist for assessment or intervention, were studied. They were given a battery of language tests and their parents completed the Children's Communication Checklist-2 (CCC-2). Results: Language tests that stressed verbal short-term memory were best at distinguishing clinically referred from other cases; narrative and vocabulary tasks were less effective. A discriminant function analysis identified a combination of language test and parental report measures as giving the best discrimination between referred and non-referred cases. Nevertheless, of 82 children classified as language impaired by the discriminant function, 44 had never been referred to a speech and language therapist. These did not appear to be false-positives; they scored at least as poorly as referred cases on literacy tests. They had significantly lower socio-economic backgrounds than referred cases. Conclusions & Implications: Language test scores provide important information about which children are at risk of academic failure, though this varies from test to test. Reliance on language tests alone, however, is insufficient; a parental report provides important complementary information in the diagnostic process. Children of low socio-economic status with language problems are particularly likely to have no contact with speech and language therapist services. (Contains 5 tables and 1 figure.)