The dynamic approach (DA) suggests that professional development should be differentiated to meet teachers' individual needs while engaging participants into systematic and guided critical reflection. Previous experimental studies demonstrated that one-year interventions based on the DA have a positive impact on teacher effectiveness. The study reported here investigates the impact that a long-term programme based on the DA can have on quality of teaching. In-service primary school teachers were randomly allocated into two groups. The first group received a three-year programme based on the DA whereas the second acted as the control group. Pre- and post-measurement of teaching skills was performed each year. Offering the DA for a longer period resulted in bigger effects on improving teaching skills but no change in the skills of the control group was observed. Differential effects for teachers situated at different levels are also reported. Implications of findings are drawn.