Purpose: The aim of this study was to describe and explain changes in severity of speech sound disorder (SSD) and token-to-token inconsistency in children with high levels of inconsistency. Method: Thirty-nine children (aged 4;6-7;11 [years;months]) with SSDs and high levels of token-to-token inconsistency were assessed every 6 months for 2 years (i.e., five assessment points). Growth modeling was used to assess relations among therapy support, receptive vocabulary, severity, and inconsistency over time. Results: Children with the most severe SSDs and highest levels of token-to-token inconsistency showed the smallest improvements in speech accuracy over time. Therapy support did not predict changes in speech accuracy or token-to-token inconsistency over time. Receptive vocabulary (measured at the outset of the study) was also a significant predictor of speech accuracy and inconsistency. Conclusions: These findings suggest that an immediate start to intervention (rather than a wait-and-see approach) is recommended for children with inconsistent speech error patterns. The results also highlight the value of developing vocabulary knowledge in addition to improving speech accuracy for some children with inconsistent speech production.