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