The study examined the confidence accuracy relationship, and the influence of child characteristics on clinician confidence, when predicting a diagnosis of Autism Spectrum Disorder during screening of 125 referred children aged under 3.5 years. The diagnostic process included observation, interview, language and developmental testing. Clinical judgement accuracy was compared against final diagnosis for high and low confidence levels (with confidence assessed on a 0–100 % scale). We identified a significant CA relationship with predictive accuracy highest at confidence levels of 90–100 %. Parent report of unusual behaviors was the only significant independent predictor of confidence. Clinicians’ confidence may be important when evaluating decisions to refer, or not to refer, children for further diagnostic assessment.