Research and clinical practices often rely on an utterance unit for spoken language analysis. This paper calls attention to the problems encountered when identifying utterance boundaries in young children's spontaneous conversational speech. The results of a reliability study of utterance boundary assignment are described for 20 females with graduate professional education in speech-language pathology. They judged audiorecorded spontaneous speech samples that were elicited from 4-year-old children. Their agreement with each other (interobserver) was significantly lower than their self-agreement (intraobserver). Interobserver agreement varied with the length/grammatical complexity of response turns, and the number and types of selective boundary cues presented in the speech stimuli. The findings have research and clinical implications for the utterance as a conceptual notion and its use constraints.