Grammatical specific language impairment (G-SLI) has been proposed as a distinct subtype of language impairment. We assessed a large sample of twins between the ages of 7 and 13 years on language comprehension tests sensitive to G-SLI. The sample included 37 same-sex twin pairs selected for the presence of language impairment (LI) in one or both twins and 104 twin pairs from the general population. The number and patterns of errors in those with LI replicated findings from previous studies of G-SLI. Qualitative markers of G-SLI were derived from the tests. Out of 144 children for whom complete data were available, 2 scored positive on all five markers and 9 scored positive on four of the five markers. Most children who made grammatical errors characteristic of G-SLI had co-occurring deficits in other areas. These results raise questions for theories that treat this disorder as an all-or-none modular deficit.