This study assessed the impact of the Lee Silverman Voice Treatment (LSVT®) on vocal loudness [sound pressure level (SPL)] in a group of dysarthric individuals with idiopathic Parkinson's disease (IPD). Pre‐ to post‐treatment changes in SPL in the treated group were compared with changes in voice SPL during the same time in two control groups: individuals with IPD not treated with the LSVT® and in non‐disordered individuals, age‐matched to the patients. All subjects produced the same voice and speech tasks—sustaining vowel phonation, reading the “Rainbow Passage,” producing a short monologue, and describing a picture. These tasks were recorded at three different occasions: just prior to treatment, just after treatment, and 6 months following treatment. The individuals treated with LSVT® increased voice SPL from baseline to post‐treatment by an average of 8 dB and from baseline to 6 months follow‐up by an average of 6 dB. These changes were statistically significant and perceptibly audible. No significant changes in SPL were observed in the control groups during the time corresponding to the treatment and follow‐up. Differences in SPL between the treated and untreated patients at post‐treatment and follow‐up were statistically significant for all voice and speech tasks. These findings, along with others, provide additional support for the efficacy of the LSVT®. Mov. Disord. 16:79–83, 2001. © 2001 Movement Disorder Society.