A 22-year-old Chinese man presented with sudden onset of generalised muscular weakness and paralysis upon awakening in the morning, due to sporadic periodic paralysis (SPP), a type of hypokalaemic periodic paralysis (HPP) without hyperthyroidism or familial history of paralysis. Laboratory studies showed marked hypokalaemia (K+1.6 mmol/litre). He received intravenous KCl supplementation at a rate of 0.14 mmol/kg/h and developed a paradoxical fall in serum K+concentration from 1.6 to 1.4 mmol/litre during KCl therapy. After 160 mmol KCl supplementation his muscular strength recovered, but muscular paralysis recurred 2 h later. Acute recurrent hypokalaemia was the presumptive initial diagnosis and intravenous KCl supplementation was briefly reinitiated. Despite no obvious abnormalities on ECG monitoring, a 12-lead ECG clearly demonstrated tented T waves in the precordial leads suggestive of hyperkalaemia, later found to be 6.9 mmol/litre. After treatment with intravenous calcium gluconate, insulin and loop diuretics, his serum K+concentration fell to 4.7 mmol/litre and muscular paralysis resolved in 3 h.