Purpose: To demonstrate the internal reliability and discriminative validity of the Language Use Inventory for Young Children (LUI; D. K. O'Neill, 2002), a newly developed parent-report measure designed to assess pragmatic language development in 18-47-month-olds. Method: To examine internal reliability, the LUI was completed by mail by 177 parents recruited from the University of Waterloo's Centre for Child Studies database, 175 of whom completed the LUI again within 4 weeks to assess test-retest reliability. To examine discriminative validity, 49 parents of children awaiting assessment at a local speech-language clinic and 49 parents of typically developing children recruited from the Centre for Child Studies database and matched in age and sex to the clinic group completed the LUI. Results: Alpha values for the subscales of the LUI were at or above acceptable levels (0.80-0.98), and steady growth in children's pragmatic language development was demonstrated. The study of discriminant validity revealed sensitivity and specificity levels over 95%. Conclusions: The LUI's internal reliability and stability were strongly supported and its sensitivity and specificity in distinguishing between typically developing and language-delayed children exceeded even the most stringent criteria of 90% accuracy.