Eight patients were seen within 15 minutes of intranasal self-administration of large amounts of pure D-lysergic acid diethylamide (LSD) tartrate powder. Emesis and collapse occurred along with signs 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 nanograms per ml and 1,000 to 7,000 mug per 100 ml, respectively. With supportive care, all patients recovered. Massive LSD overdose in man is life-threatening and produces striking and distinctive manifestations.