Autistic Spectrum Disorder (ASD) and Specific Language Impairment (SLI) are disorders of communication that are sometimes thought to show similar structural language difficulties. Recent research has even suggested that they might be aetiologically related. However, it may be that standardized language tasks are not sensitive enough to detect similarities and differences accurately. This study involved 26 Greek children with either ASD or SLI and compared them on standardized measures of structural and pragmatic language as well as using a structured narrative task. Children with ASD were more impaired on receptive but not expressive scores from standardized language tests. In contrast, narrative measures showed significantly poorer ASD performance in expressive skills involving wider story-telling skill and in some sentence-level skills, in particular referencing, compared to peers with SLI. ASD and SLI groups also showed different relationships between structural language and other measures. The data suggests that narrative is a useful tool for revealing qualitative differences in language between overlapping communication disorders both at the clinical and theoretical level, since it provides information that is lost in more formalized testing. This may be particularly true where norms are not available or testing is difficult. (Contains 5 tables and 1 figure.)