Purpose: The primary purpose of this study was to determine the strength of interspeaker and intraspeaker articulatory-to-acoustic relations of vowel contrast produced by talkers with dysarthria and controls. Methods: Six talkers with amyotrophic lateral sclerosis (ALS), six talkers with Parkinson's disease (PD), and 12 controls repeated a sentence at typical, slow, and fast rates. Tongue displacements and acoustic vowel distances were measured to determine articulatory and acoustic vowel contrasts. Results: Interspeaker articulatory-to-acoustic relations were strong for talkers with PD and controls but weak for talkers with ALS and controls. Further, predominantly moderate and strong intraspeaker articulatory-to-acoustic relations were found in response to rate modulations; however, correlation coefficients were significantly lower in talkers with ALS than in controls. Conclusions: The findings on interspeaker articulatory-to-acoustic relations suggested that the degree of tongue displacement can be accurately inferred from the degree of acoustic vowel contrast in talkers with PD but not in talkers with ALS. Findings on intraspeaker articulatory-to-acoustic relations generally supported the longstanding notion that speaking rate-induced changes in tongue displacement evoke similar changes in acoustic vowel contrast. Differential effects of the pathophysiology on inter- and intraspeaker articulatory-to-acoustic relations are discussed.