This study explored the effect of thiamine deficiency during early infancy on the development of syntax and lexical retrieval. We tested syntactic comprehension and production, lexical retrieval abilities and conceptual abilities of 59 children aged 5-7 years who had been fed during their first year of life with a thiamine-deficient milk substitute. We compared them to 35 age-matched control children who were fed with other milk sources. Experiment 1 tested the comprehension of relative clauses using a sentence-picture-matching task. Experiment 2 tested the production of relative clauses using a preference elicitation task. Experiment 3 tested the repetition of various syntactic structures with various types of syntactic movement and embedding. Experiment 4 tested picture naming and Experiment 5 tested lexical substitutions in a sentence repetition task. Experiments 6 and 7 tested the children's conceptual abilities using a picture association task and a picture absurdity description task. The results indicated a very high rate of syntactic and lexical retrieval deficits in the group of children who were exposed to thiamine deficiency in early infancy: 57 of the 59 thiamine-deficient children examined had language impairment, compared with three of the 35 controls (9%). Importantly, unlike the impairment this group sustained in their language abilities, the conceptual abilities of most of the children were intact (only six children, 10%, were conceptually impaired). These findings indicate that thiamine deficiency in infancy causes severe and long-lasting language disorders and that nutrition may be one of the causes for language impairment.