Purpose: This report investigated the validity of using percent grammatical responses (PGR) as a measure for assessing grammaticality. To establish construct validity, we computed the correlation of PGR with another measure of grammar skills and with an unrelated skill area. To establish concurrent validity for PGR, we computed the correlation of PGR with a previously validated measure of grammaticality, percent grammatical utterances (PGU), and examined the extent to which PGR and PGU agreed upon pass/fail decisions for children. Method: Participants included 79 3-year-olds from mostly middle socioeconomic status homes. Language samples were elicited by asking children to describe 15 pictures in response to 4 questions per picture. To calculate PGU, children's responses to all 4 questions were segmented into communication units, and each communication unit was evaluated for grammatical errors. To calculate PGR, the entire response to just the first question was evaluated for grammatical errors. Results: PGR scores significantly correlated with a standardized test of grammar (r = 0.70), but not with a measure of vocabulary (i.e., type-token ratio; r = 0.11). In addition, PGR scores were significantly correlated with PGU scores (r = 0.88). Agreement between PGR and PGU was 92% for pass decisions and 94% for fail decisions. Conclusions: The current study establishes the construct validity of PGR as a measure of grammar and supports the use of PGR as a measure to assess grammaticality.