PURPOSE: The present study examined associations of 5 endophenotypes (i.e., measurable skills that are closely associated with speech sound disorders and are useful in detecting genetic influences on speech sound production), oral motor skills, phonological memory, phonological awareness, vocabulary, and speeded naming, with 3 clinical criteria for classifying speech sound disorders: severity of speech sound disorders, our previously reported clinical subtypes (speech sound disorders alone, speech sound disorders with language impairment, and childhood apraxia of speech), and the comorbid condition of reading disorders. PARTICIPANTS AND METHOD: Children with speech sound disorders and their siblings were assessed at early childhood (ages 4-7 years) on measures of the 5 endophenotypes. Severity of speech sound disorders was determined using the z score for Percent Consonants Correct-Revised (developed by Shriberg, Austin, Lewis, McSweeny, & Wilson, 1997). Analyses of variance were employed to determine how these endophenotypes differed among the clinical subtypes of speech sound disorders. RESULTS AND CONCLUSIONS: Phonological memory was related to all 3 clinical classifications of speech sound disorders. Our previous subtypes of speech sound disorders and comorbid conditions of language impairment and reading disorder were associated with phonological awareness, while severity of speech sound disorders was weakly associated with this endophenotype. Vocabulary was associated with mild versus moderate speech sound disorders, as well as comorbid conditions of language impairment and reading disorder. These 3 endophenotypes proved useful in differentiating subtypes of speech sound disorders and in validating current clinical classifications of speech sound disorders.