Dynamic testing aims to explore a child’s potential to learn by assessing improvement after training. In this study we investigated the relationship between performance on a dynamic test of series completion and children’s cognitive flexibility. This was done using a pre-test-trainingpost-test control-group design with 95 children, aged 6-8 years (M = 7;1, SD = 12.5 months). All children were tested with a measurement of cognitive flexibility. Half of the children were trained in series completion according to a graduated prompting model, while the other half only practiced. Based on initial ability and performance change after training, children were classified as non-learner, learner or high performer. The results showed that training improved series completion performance more than practice-only. Cognitive flexibility predicted static pre-test performance and instructional needs during training and might therefore be of importance in the assessment of learning potential.