Conversational indices of language impairment were used to investigate similarities and differences among children with Attention-Deficit/Hyperactivity Disorder (ADHD), children with Specific Language Impairment (SLI) and children with typical development (TD). Utterance formulation measures (per cent words mazed and average number of words per maze) differentiated the ADHD group from the SLI and TD groups (ADHD is greater than TD=SLI). In contrast, measures of lexical diversity, average sentence length and morphosyntactic development (number of different words, MLU, and composite tense) differentiated the SLI group from the ADHD and TD groups (SLI is less than ADHD=TD). High levels of within group variation were observed in children's speaking rate (words per minute). Implications for differential diagnosis and the establishment of phenotypes for developmental language disorders are discussed.