Eight patients were seen within 15 min of intranasal self-administration of large amounts of pure D-lysergic acid diethylamide (LSD) tartrate powder. Emesis and collapse occurred along with sign of sympathetic overactivity, hyperthermia, coma, and respiratory arrest. Mild generalized bleeding occurred in several patients and evidence of platelet dysfunction was present in all. Serum and gastric concentrations of LSD tartrate ranged from 2.1 to 26 ng/ml and 1000 to 7000 mug/100 ml, respectively. With supportive care, all patients recovered. Massive LSD overdose in humans is life-threatening and produces striking and distinctive manifestations.