Previous work has suggested that, because writing is a late-acquired and complex skill, it may be a particularly sensitive index of language difficulties in children. Evidence in support of this view was obtained in a study contrasting 161 normally-developing control children aged from 7.5 to 13 years with 75 twin children of the same age who either had specific speech-language impairments, or were co-twins of affected children. Written narratives were elicited from children using a sequence of five photographs depicting a simple story, and were analysed for grammatical complexity and accuracy, intelligibility, and semantic content. Only 42 of the twins could spell well enough to attempt the narrative task. Some co-twins of affected children had deficits in written language, despite normal performance on oral language tests. Most children with language impairments were poor at writing, with particularly marked deficits on a measure of spelling and punctuation. Children with language impairments made a relatively high proportion of phonologically inaccurate spelling errors when compared with younger children at a similar vocabulary level. Those who did poorly on a nonword repetition test were especially likely to have poor written language. However, four children with pure speech difficulties produced age-appropriate written narratives.