This study examined whether children's use of subject relative clauses differs as a function of their English dialect (African American English, AAE vs. Southern White English, SWE vs. Mainstream American English, MAE) and clinical diagnosis (specific language impairment, SLI vs. typically developing, TD). The data were spontaneous language samples from 87 AAE- and 53 SWE-speaking children, aged 3 to 6 years. Data on MAE came from previously published studies. Results were that the TD child speakers of AAE and SWE presented similar rates and types of subject relative clauses within their samples, but the rates at which they supplied the relative marker within these clauses varied from those that have been reported for TD child speakers of MAE. Nevertheless, across both AAE and SWE, the rates at which the children with SLI produced relative markers within clauses were lower than the rates of their TD peers, and these findings could not be explained by differences in the children's overall rates of non-mainstream English pattern use. These findings are consistent with studies of MAE-speaking children, and they also show across-dialect similarities in the grammatical deficits of children with SLI. (Contains 5 tables.)