This article argues that the patterns governing the retention and elision of Proto-Uto-Aztecan (PUA) *p in the language groups Nahuan and Corachol can be explained by reference to accent patterns in the most recent shared ancestor Proto-Corachol-Nahua (PCN). The PCN accent can be reconstructed by application of Sapir’s law of accent in Nahuan, which posits that accent in pre-Nahuan can be predicted from the patterns of syncope. Comparison shows that the pre-Nahuan accent coincides with lexical accent in Corachol. It is argued that in both language groups *p is retained in accented syllables and weakened to *h (and then elided) in unaccented syllables. Accent appears to interact with the Uto-Aztecan system of consonant gradation. The reconstruction of accent and its effects in Corachol and Nahuan and other SUA languages may permit a better understanding of the relations between the languages and of phenomena that may otherwise appear to be irregular.