The aim of this study was to establish the construct validity of the Focus on the Outcomes of Communication Under Six (FOCUS©). This measure is reflective of concepts in the International Classification of Functioning Disability and Health - Children and Youth framework. It was developed to capture 'real-world' changes (e.g. communicative participation) in preschoolers' communication following speech-language intervention. Method: A pre-post design was used. Fifty-two parents of 3- to 6-year-old preschoolers attending speech-language therapy were included as participants. Speech-language therapists provided individual and/or group intervention to preschoolers. Intervention targeted: articulation/phonology, voice/resonance, expressive/receptive language, play, and use of augmentative devices. Construct validity for communicative participation was assessed using pre-intervention and post-intervention parent interviews using the FOCUS© and the communication and socialization domains of the Vineland Adaptive Behavior Scales-II (VABS-II). Results: Significant associations were found between the FOCUS©, measuring communicative participation, and the VABS-II domains for: (i) pre-intervention scores in communication (r = 0.53, P < 0.001; 95% CI 0.30-0.70) and socialization (r = 0.67, P < 0.001; 95% CI 0.48-0.80); (ii) change scores over-time in communication (r = 0.45, P < 0.001; 95% CI 0.201-0.65) and socialization (r = 0.39, P = 0.002; 95% CI 0.13-0.60); and (iii) scores at post-intervention for communication (r = 0.53, P < 0.001; 95% CI 0.30-0.70) and for socialization (r = 0.37, P = 0.003; 95% CI 0.11-0.50). Conclusions: The study provided evidence on construct validity of the FOCUS© for evaluating real-world changes in communication. We believe that the FOCUS© is a useful measure of communicative participation.