This study explored relationships between vocabulary size and self-regulation and joint engagement in 28 children with multi-system developmental delay (DD) aged 2;5 (years;months) to 5;6 and a language age-matched control group of 28 typically developing (TD) children aged 0;7 to 5;6 drawn from a larger sample of 77. Parents completed the ABASII, Second Edition (ABASII; Harrison & Oakland, 2003), with the Leisure, Self-direction, and Social subtests serving as measures of self-regulation and joint engagement. Vocabulary size was measured using an adaptation of the New Zealand version of the MacArthur Communicative Development Inventory: Toddlers (CDI; Reese & Read, 2000). Responses to the Language Use Inventory (O'Neill, 2007) were also collected for comparison with the CDI. Group differences on vocabulary size and the ABASII Social and Self-direction subtests were not significant. However, children with multi-system DD scored significantly higher on the Leisure subtest. Data from the children with multi-system DD revealed a medium, positive correlation between the CDI total score and the raw score of the Leisure subtest, r = 0.34, p = 0.075 and for the TD children a strong, positive correlation r = 0.51, p = 0.006. For the children with multi-system DD, there was a medium, positive correlation between the CDI total score and the raw score of the Self-direction subtest, r = 0.39, p = 0.038 and a strong, positive correlation for the TD children, r = 0.52, p = 0.005. Similarly, for the children with multi-system DD there was a medium, positive correlation between the CDI total score and the raw score of the Social subtest, r = 0.41, p = 0.032 and a strong, positive correlation for the TD children, r = 0.63, p < 0.001. The results suggest a positive correlation between self-regulation and joint engagement and vocabulary development in both groups of children.