The authors report a remarkable glucose lowering side effect of the often used drug hydroxychloroquine (Plaquenil) in an otherwise healthy non-diabetic patient, who presented with severe hypoglycaemia. Although the mechanism has not been clarified yet, increased insulin sensitivity as well as decreased insulin degradation is suggested to contribute to the reduction in serum glucose levels. Clinicians prescribing this drug must be warranted and check glucose levels in the initial phase. Moreover, hydroxychloroquine might be applied as therapeutic in refractory diabetic rheumatics or rheumatic patients at risk to develop diabetes mellitus.