Pseudohyperkalaemia is an uncommon and frequently unrecognised biochemical abnormality. It occurs as a consequence of aggregation and lysis of platelets in vitro. As a result, potassium is released, which causes an elevated serum concentration. We present the case of a 21-year-old man with a traumatic splenic injury necessitating laparotomy and splenectomy. Following surgery he developed hyperkalaemia. Further investigations diagnosed pseudohyperkalaemia, one of the causes of which is thrombocytosis secondary to splenectomy.