Although the adoption design is the most powerful method to disentangle nature and nurture, it has not been applied previously to developmental speech or language disorders. The present study examined the speech outcomes of 156 adopted and nonadopted children at varying risk for speech disorders based upon self-reported parental speech history. The sample consisted of four groups: (a) 16 adopted children with an affected biological parent; (b) 19 adopted children with an affected adoptive parent; (c) 31 nonadopted children with an affected natural parent; and (d) 90 low-risk adopted and nonadopted children with no parental speech disorder history. Results revealed that 25% of the children with a genetic background of speech disorder displayed questionable speech, language, or fluency skills at age 7, in comparison to 9% of the children with no known genetic history. Logistic regression analyses indicated that positive biological parental background was the best predictor of offspring affected status. The child's Full-Scale IQ and the HOME Scale of family environment were not significantly associated with speech outcome. These results provide additional evidence that genetic factors contribute importantly to the vertical transmission of some developmental speech disorders of unknown origin.