Grade 6 teachers (N = 106) in one school district were randomly assigned to early or late professional development (PD) groups. The program focused on reform communication and incorporated principles of effective PD recommended by researchers, although the duration of the treatment was modest (one full day and four after school sessions over a ten-week period). At the post-test, there were no statistically significant differences in student achievement. Although it could be argued that the result demonstrates that PD resources should be redirected to more intensive PD delivered over longer periods, we claimed that the PD was assessed prematurely. After the completion of the study, the external assessments administered by the province showed a significant increase in student achievement from one year to the next involving both the early and late treatment groups, an increase that was not found for the same students in other subjects. The study had high ecological validity: it was delivered by district curriculum staff to all grade 6 teachers, volunteers and conscripts alike. The cost to the district, less than CAN$14 [9 euros] per student, was comparable to the modest expenditures typically available for professional development in Canadian school districts.