Introduction  Studies in the fields of voice and speech have increasingly focused on the vocal tract and the importance of its structural integrity, and changes in the anatomy and configuration of the vocal tract determine the variations in phonatory and acoustic measurements, especially in the formation of the formants (Fs). Recent studies have revealed the functional consequences arising from being overweight and having an accumulation of fat in the pharyngeal region, including obstructive sleep apnea syndrome (OSAS) and impacts on the voice. Objectives  To assess the relationship between body mass index (BMI) and analysis of the speech. Methods  This study was approved by the Ethics Committee of the Universidade Federal de São Paulo (no. 288,430). The cohort consisted of 124 individuals aged between 18 and 45 with full permanent dentition and selected randomly. The participants underwent a brief medical history taking, BMI assessments and recording emissions of the sustained vowels /a/, /ε/, /i/, and /u/ by acoustic program PRAAT (v. 5.3.85, Boersma and Weenink, Amsterdam, Netherlands). Recordings were taken using a unidirectional microphone headset (model Karsect HT-9, Guangdong, China), with a condenser connected to an external sound card (USB-SA 2.0, model Andrea, PureAudio™, Pleasant Grove, UT, USA), to reduce noise. Results  There was a significant correlation between BMI and formant 3 (F3) vowel /a/; however, there was a low degree of correlation intensity. Conclusions  We did not observe a correlation between the BMI and the speech formants, but we believe there is a trend in this correlation that leads to changes in speech patterns with increases in BMI.