Vowel inventories and error patterns for three children with suspected developmental apraxia of speech (DAS) were analysed over a 3-year period using phonetic transcriptions of connected speech samples. The children demonstrated complete English vowel inventories except for rhotics. However, accuracy of vowel targets in connected speech did not normalize during this period. No consistent patterns of vowel errors were found. There was no decrease in vowel accuracy with increased utterance length or syllable complexity. Findings indicate persistence of vowel inaccuracy in the presence of a relatively complete vowel inventory. This longitudinal data can help to build consensus on use of vowels as a potential clinical marker for differential diagnosis of DAS.