Purpose: Ecologically realistic, spontaneous, adult-directed, longitudinal speech data of young children were described by acoustic analyses. Method: The first 2 formant frequencies of vowels produced by 6 children from different American English dialect regions were analyzed from ages 18 to 48 months. The vowels were from largely conversational contexts and were classified according to dictionary pronunciation. Results: Within-subject formant frequency variability remained relatively constant for the span of ages studied. It was often difficult to detect overall decreases in the first 2 formant frequencies between ages 30 and 48 months. A study of the movement of the corner vowels with respect to the vowel centroid showed that the shape of the vowel space remained qualitatively constant from 30 through 48 months. Conclusions: The shape of the vowel space is established early in life. Some aspects of regional dialect were observed in some of the subjects at 42 months of age. The present study adds to the existing data on the development of vowel spaces by describing ecologically realistic speech.