Purpose The Lidcombe Program is an efficacious and effective intervention for early stuttering. The treatment is based on parent verbal response contingent stimulation procedures, which are assumed to be responsible for treatment effect. The present trial tested this assumption. Method The design was a parallel, open plan, noninferiority randomized controlled trial. In the experimental arm, the five Lidcombe Program verbal contingencies were removed from parent instruction. The primary outcome was beyond-clinic percentage syllables stuttered at 18-month follow-up. Seventy-four children and their parents were randomized to one of the two treatment arms. Results Findings of noninferiority were inconclusive for the primary outcome of stuttering severity, based on a margin of 1.0 percentage syllables stuttered. Conclusions The inconclusive finding of noninferiority means it is possible that verbal contingencies make some contribution to the Lidcombe Program treatment effect. However, considering all primary and secondary outcomes, an overriding impression from the trial is a similarity of outcomes between the control and experimental arms. The clinical applications of the trial are discussed, along with further research that is needed.