Vowel discrimination ability was observed in two groups of 20 hearing-impaired subjects each. Each group listened to a different list of closed-set test items specifically designed for the study. A surprisingly low number of errors occurred, suggesting that vowel items in general lack the efficiency required for speech discrimination testing using a closed-set response system. Among the most difficult phonemes to discriminate clearly were /ɔ I /, /ɔ/, /au/, /ε/, /o/, and /α/. The phonemes most frequently substituted in error were adjacent to the stimulus phoneme on the Formant 1 versus Formant 2 vowel charts. The /u/ was the most frequent substitution for several vowels.