AbstractThe syntactic and interactive language development of a single twin dyad was investigated longitudinally. One of the dyad was language‐impaired and received therapy. The other child developed essentially normal language. This study was conducted over a nineteen‐month period and examined the effects of the twin situation on language form and function. The language sample was the basis for analysis and separate attention was given to each twin alone with the experimenter and then to the twins' interaction with each other and the experimenter in order to examine patterns of conversational dominance. Results indicated that the presence of the normal twin had a depressing effect on certain language measures of the language‐impaired twin but that this effect became less marked after the institution of th