"Background": Clinical trials have shown that behavioural treatments based on variants of prolonged-speech (PS) are best practice for reducing the stuttering rate in adults. However, while stuttering is significantly reduced or eliminated for most adults in the short-term with such treatment, relapse in the longer-term is common. Consequently, there has been interest in attempting to establish variables that predict responsiveness to PS-based treatments. Identifying such variables would enable risk prediction and also contribute to the tailoring of treatments to suit individuals who are less likely to benefit in the long-term from the conventional PS-based treatments. Variables that have been investigated to date are the stuttering severity, the attitude to communication and the locus of control. "Aims": The present study revisited this issue with methodological improvements. "Methods & Procedures": Prospective, continuous measures of outcome several years after treatment, and well-powered, least-squares multivariate regression, with backwards elimination, were used to determine the best model to predict short-and long-term outcomes of a PS-based treatment. Other predictor variables were also included. The stuttering rates of 78 participants were measured immediately after the intensive stage of treatment, and during a surprise telephone call 3.5-5 years after treatment. "Outcomes & Results": Attitude to communication and locus of control were separate constructs to stuttering rate. While the stuttering rate predicted outcome, neither of the two non-behavioural variables predicted outcome in either the short-or the long-term. "Conclusions": The most significant finding is that the non-behavioural variables of attitude to communication and locus of control did not predict treatment outcomes. This is contrary to the findings of most previous studies. (Contains 5 tables.)