Children with unintelligible speech differ in severity, underlying deficit, type of surface error patterns and response to treatment. Detailed treatment case studies, evaluating specific intervention protocols for particular diagnostic groups, can identify best practice for children with speech disorder. Three treatment case studies evaluated the efficacy of Core Vocabulary intervention for three boys with inconsistent speech disorder. The cases examined the effects of previous intervention, use of default preferred word plans and behaviour disorder on intervention outcome. Inconsistent speech disorder was diagnosed after Diagnostic Evaluation of Articulation and Phonology assessment. A Core Vocabulary approach to intervention was selected to focus on planning whole word production rather than surface error patterns or specific sound features. Individual differences between cases led to different amounts of intervention and the number of words taught during intervention. All three boys showed gains in intelligibility, accuracy and consistency of word production. Core Vocabulary intervention was shown to be appropriate for all three children, although their individual differences required clinical adaptation of the approach.