The need for a standardized approach to the selection of research subjects for studies on language impairment has long been an area of controversy in the literature. The data obtained from the selection of language-impaired and control subjects for the San Diego Longitudinal Study allows one to evaluate the effects a two-stage subject-selection procedure has on the characteristics of the language-impaired and normal subjects selected for research studies. Specifically, what effect do quantitative measures, as compared to clinical judgements and referrals, have on the characteristics of subjects selected for research studies? Explicit and detailed oral and written descriptions of subjects sought for both a language-impaired group and a control group were given to teachers and clinicians. However, only 39 of the first 100 children referred as language-impaired and 29 of the first 60 referred as normal controls were found on standardized testing to meet the study criteria and to be matched on important variables, such as IQ and socioeconomic status. These results demonstrate the importance of establishing quantitative inclusionary as well as exclusionary criteria for selecting and matching subjects in clinical research studies.