The effect of upper respiratory tract diseases on phonation has been reviewed, but little is known about the influence of lower respiratory tract diseases. In particular, the effect of asthma as a reversible obstructive small-airway disease on phonatory variables is not yet clear. We conducted a cross-sectional controlled study to evaluate the quality of phonation in a group of 34 adults with untreated mild to severe persistent asthma who were seen at the Ghaem Hospital in Mashhad, Iran. Patients with sinusitis, gastroesophageal reflux disease, or primary laryngeal disease were ineligible for study participation. For comparison purposes, we identified a group of nonasthmatic, age- and sex-matched healthy controls. We evaluated eight voice parameters: basal voice frequency at the glottic level (F0), jitter, shimmer, breathiness, harshness, hoarseness, normalized noise energy (NNE), and S/Z ratio. These parameters were measured by a voice meter with Dr. Speech statistical software. We found that values for F0, jitter, and shimmer were very similar in the two groups, but there were statistically significant differences in values for harshness, hoarseness, NNE, S/Z ratio (all p < 0.01), and breathiness (p = 0.015). Our findings suggest that lower airway diseases such as asthma can impair phonation, and we recommend future studies with larger populations to further explore this issue.