['The authors describe the procedures used to explain an unexpected finding that adolescents with Down syndrome (DS) had a lower mean length of utterance (MLU) than typically developing (TD) children in interviews without picture support, but not in narratives supported by wordless picture books. They hypothesize that the picture support of the narrative context increased the MLU for the group with DS alone.', 'Adolescents with DS (n = 14) and TD children (n = 14) matched for receptive syntax narrated picture storybooks and participated in interviews. Transcription reliability, intelligibility/fluency, grammatical errors, discourse and sampling contexts, and discourse characteristics were examined for their effects on MLU.', 'The DS group showed a greater responsiveness to adult questions than the TD group; an alternate MLU without yes/no responses showed the same interaction of group and context as the original finding. An additional comparison of MLUs, obtained from narratives present in the interview and narratives elicited using picture books, showed that picture support in narrative increased MLUs only for the group with DS.'] Does sampling context affect MLU in the language of adolescents with Down syndrome?