Most intelligibility tests are based on the use of monosyllabic test stimuli. This constraint eliminates the ability to measure the effects of lexical stress patterns, complex phonotactic organizations, and morphological complexity on segmental intelligibility. Since these aspects of lexical structure affect speech production (e.g., changing syllable duration), it is likely that they affect the structure of acoustic-phonetic patterns. It also seems plausible that listeners make use of this knowledge during segmental perception. Thus, to the extent that text-to-speech systems fail to modify acoustic-phonetic patterns appropriately in polysyllabic words, intelligibility may suffer. This means that while most standard intelligibility tests may accurately estimate segmental intelligibility in monosyllabic words, this estimate may not generalize prediction of segmental intelligibility with more complex lexical forms. The present study was carried out to measure segmental intelligibility in stimuli varying in lexical complexity. Monosyllabic, bisyllabic, and polysyllabic words were used varying in morphological complexity (monomorphemic or polymorphemic). Listeners transcribed these stimuli spoken by two human talkers and two text-to-speech systems varying in speech quality. The results indicate that lexical complexity does affect the measured segmental intelligibility of synthetic speech.