Combination ribociclib and aromatase inhibitors are currently the preferred treatment in Australia for newly diagnosed hormone receptor positive metastatic breast cancer in the absence of visceral crisis. In our case series of 32 patients, 28% experienced grade 1 elevations in creatinine, a toxicity that was under-recognised in large phase III studies. Creatinine rise appears to be due to a reversible inhibition of renal efflux transporters rather than an acute kidney injury in the majority of cases.