This study investigated the relationship between scores on standardized tests (Clinical Evaluation of Language Fundamentals [CELF], Peabody Picture Vocabulary Test-Third Edition [PPVT-III], and Expressive Vocabulary Test) and measures of spontaneous speech (mean length of utterance [MLU], Index of Productive Syntax, and number of different word roots [NDWR]) derived from natural language samples obtained from 44 children with autism between the ages of 4 and 14 years old. The children with autism were impaired across both groups of measures. The two groups of measures were significantly correlated, and specific relationships were found between lexical-semantic measures (NDWR, vocabulary tests, and the CELF lexical-semantic subtests) and grammatical measures (MLU, and CELF grammar subtests), suggesting that both standardized and spontaneous speech measures tap the same underlying linguistic abilities in children with autism. These findings have important implications for clinicians and researchers who depend on these types of language measures for diagnostic purposes, assessment, and investigations of language impairments in autism.