Children with specific language impairment (SLI) demonstrate consistent comprehension problems. The present study investigated whether these problems are driven primarily by structural complexity or length. A picture-sentence matching task was presented to 30 children: (1) 10 children with SLI, (2) 10 comprehension-matched children with typical language development (TLD) and (3) 10 children with TLD matched for chronological age. Argument-structure complexity was manipulated independently of length, which was also independently varied. Results showed that argument-structure complexity had a greater influence on comprehension in children with SLI than in the comparison groups, with transitive sentences eliciting more errors than intransitive ones. This effect was not dependent on sentence length, which did not appear to affect the comprehension level. The results support the view that comprehension problems in children with SLI are principally related to the structural complexity of the sentence rather than the amount of material to be processed. (Contains 4 tables, 1 figure and 1 note.)