Hyperkalemia paralysis is a very rare disorder that can be easily managed by the adjustment of serum potassium levels. It is characterised by an ascending flaccid motor weakness and differential diagnosis, mainly Guillan-Barré syndrome, should be considered. We describe a case of ascending flaccid paralysis in a patient admitted to an emergency unit in Sao Paulo, Brazil. The patient had previously received Spironolactone and examination revealed hyperkalemia and chronic renal failure due to a polycystic kidney disease. With the improvement in his potassium levels, the paralysis disappeared until complete remission occurred. This report indicates another important and poorly recognised manifestation of hyperkalemia.