Background: There is empirical evidence that a developmental language disorder (DLD) in early childhood leads to behaviour problems. However, it is still not clear how changes in language proficiency in these children influence the presence of behaviour problems. Aims: The aim of this study is to examine if changes in language proficiency are related to changes in behaviour problems in toddlers indicated to have DLD. Methods & Procedures: This study included 185 toddlers indicated to have DLD (mean age 38 months at pretest). Scores on receptive and expressive language domains and internalizing and externalizing behaviour were gathered on Wave 1 and Wave 2 using Routine Outcome Monitoring. The Reliable Change Index was used to categorize children into two groups: children improving in receptive and expressive language domains and children not improving. Outcomes & Results: For receptive syntax, receptive vocabulary and expressive syntax, 30% or less of the children improved. Only for expressive vocabulary, most children improved (63%). Behaviour problems were present in 17% (internalizing) and 23% (externalizing) of the children. Changes in language proficiency did not lead to changes in internalizing or externalizing behaviour problems, not for the total sample, nor for children displaying behaviour problems at Wave 1. Conclusions & Implications: Professionals working with toddlers indicated to have DLD need to be aware of the co-occurrence of language problems and behaviour problems, and have to realize that behaviour problems might not immediately decrease when language proficiency improves. If behaviour problems are present in toddlers indicated to have DLD, interventions should not only focus on language, but also on behaviour problems.