We investigated infant precursors of low language scores in early childhood. The sample included 373 probands in 130 monozygotic (MZ) and 109 same-sex dizygotic (DZ) twin pairs in which at least one member of the pair scored in the lowest 15th percentile of a control sample on a general language factor derived from tester-administered tests at 4;6. From data at 2;0, 3;0 and 4;0 the antecedents of poor language performance at 4;6 for these probands were compared to 290 control children. As early as 2;0, language measures substantially predicted low-language status at 4;6, with predictions increasing at 3;0 and 4;0. Nonverbal cognitive development at 3;0 and 4;0 was nearly as predictive of low language at 4;6 as were the language measures. Behaviour problems were also significant predictors of low language status although the associations were only about half as strong. Bivariate genetic analyses indicated that these predictions are mediated by both genetic and shared environmental links.