The objective of this study was to investigate the effect of surgical rapid maxillary expansion (SRME) on vowel production. The subjects included 12 patients, whose speech were considered perceptually normal, that had undergone surgical RME for expansion of a narrow maxilla. They uttered the following Turkish vowels, ([a], [[epsilon]], [[inverted m]], [i], [[reversed c]], [oe], [u], [y]), in isolation before and after the maxillary expansion. These samples were recorded for acoustical analysis. The fundamental frequencies (F[subscript 0]) and the frequencies and bandwidths of the first two formants (F[subscript 1] and F[subscript 2]) of the vowels were measured using the Multi-Speech programme (Kay Elemetrics). Statistical analysis revealed that the second formant frequency (F[subscript 2]) and bandwidth values in the vowel [i] and [oe] were lowered after maxillary expansion (p less than 0.05). Surgical RME induced the decrease in the second formant of the [i] and [oe] vowels by affecting the size of the anterior oral cavity, however the influence was too small to cause any differences in the acoustic properties of the other vowels. (Contains 3 tables and 4 figures.)