This study reports on the effects of a 1-year parental involvement intervention on reading development of primary school children. The intervention included a teacher and parent training, focusing on teacher-parent communication and parental involvement in homework and reading at home. We used a cluster randomized controlled trial design with pre-, post- and follow-up measurements to evaluate the intervention. Participants were 600 second grade children (n = 264 in the intervention group, n = 336 in the control group), with their parents and teachers from 24 schools in Dar Es Salaam, Tanzania. A hierarchical linear model, which assessed children's reading growth across three time points, demonstrated that compared to children in the control condition, children in the intervention condition made significantly more progress in decoding skills, reading fluency and reading comprehension from pre- to post-intervention, and from pre-intervention to follow-up (8-9 months after the intervention). We discuss the usefulness of this intervention in helping children who are at risk for reading problems in primary school.