Predictors of expressive grammar were compared in formal and naturalistic assessment tasks for children with typically developing (TD) language and with Developmental Langauge Disorder (DLD). Standardized expressive language assessments were administered to 110 preschoolers. The parents of these children reported whether or not they were concerned about their child's speech and language development. Stepwise regression analyses revealed receptive language as the only significant predictor of expressive grammar across assessment tasks. For TD preschoolers, receptive vocabulary and grammar accounted for expressive grammar performance in the formal task; however, only receptive grammar accounted for performance in the naturalistic task. For DLD preschoolers, only receptive vocabulary accounted for expressive grammar performance across both tasks. Nonverbal IQ and parent concern did not predict expressive grammar performance in either task. Implications for treatment of preschool DLD using relative strengths in vocabulary are discussed.