Purpose: In the two experiments in this study, the author examined the effects of increased vocal effort (loud speech) and amplification on sentence and word intelligibility in speakers with Parkinson disease (PD). Methods: Five talkers with PD produced sentences and words at habitual levels of effort and using loud speech techniques. Amplified sets of sentences and words were created by increasing the intensity of habitual stimuli to the level of loud stimuli. Listeners rated the intelligibility of the 3 sets of sentences on a 1-7 scale and transcribed the 3 sets of words. Results: Both loud speech and amplification significantly improved intelligibility for sentences and words. Loud speech resulted in greater intelligibility improvement than amplification. Conclusions: By comparing loud and amplified scores, about one third to one half of intelligibility improvement with loud speech could be attributed to increases in audibility or signal-to-noise ratio. Thus, factors other than increased intensity must be partly responsible for the loud speech benefit. Changes in articulation appear to play a relatively small role: Initial /h/ was the only consonant to consistently show improvement with loud speech. Phonatory changes such as improvements in F[subscript 0] and spectral tilt may account for improved speech intelligibility using loud speech techniques. (Contains 4 figures and 2 tables.)