In previous work signal processing algorithms were presented for real-time mapping of vowels to a two-dimensional display for articulation training [Beck and Zahorian, ICASSP-92, II, 241–244]. In this paper, a series of tests designed to evaluate the effectiveness of this display for vowel training are reported. Experiments were conducted with five non-native normally hearing speakers of English and with four hearing-impaired children. The foreign speakers practiced with the vowel training aid for two 30 min sessions per week over a 10 week period. Pre, mid, and post-training testing showed that all subjects learned to modify vowel productions to consistently match the specified ‘‘target’’ vowel positions depicted on the computer. The vowel training exercises with hearing-impaired school children (ages 7 to 10) were performed in therapy sessions over a 1-year period. The quantitative evaluation of the children’s data is more ambiguous than for the foreign-speaker training, but there was some progress. These results show that the instantaneous visual feedback provided by the vowel training system can be used to improve vowel productions, but that additional improvement is still required to make the system more effective for hearing-impaired children.