Pupil referral units (PRUs) in Wales accommodate children who present with a range of difficulties that cannot be managed within a mainstream setting. Many children attending PRUs in Wales do not develop the numeracy skills that they need to support their learning across the curriculum. In an effort to teach and assess addition skills, the authors assessed the effects of using a combination of direct instruction (DI) and precision teaching (PT) in a PRU. Over six school weeks, we worked with five children (aged 7 to 10 years) on a 1:1 basis through the Corrective Mathematics addition curriculum (Engelmann and Carnine, 2005). Following each lesson, the children completed an individualised fluency assessment, which we tailored to their needs using PT methods. We collected baseline and follow- up data using the Test of Early Mathematics Ability (TEMA-3), the Wide Range Achievement Test (WRAT-4) and the Corrective Mathematics placement test. We also interviewed the children post- intervention to gain insight into their experience of the approach. The results provide evidence to support the use of an instructional fluency approach in a PRU setting to help children develop early mathematics skills, particularly for children who engaged in the sessions regularly. Due to the small sample size, the results of this study have limited generalisability but may help shape future research investigating effective strategies for teaching mathematics in PRUs.