The administration of magnesium sulphate is a proposed novel therapy for Irukandji syndrome 1. In this nonrandomized, unblinded case series, data from ten patients who received magnesium salts are reviewed. Magnesium sulphate boluses of 10 to 20 mmol, in the six patients for which there was adequate data, reduced pain scores immediately after administration from 8.7±1.5 to 2.8±2.8 (Wilcoxon rank-sum test, P=0.03). In ten patients blood pressure decreased with a mean difference of-18 mmHg in mean arterial pressure. Magnesium requirements in individual patients varied markedly. Pain on injection occurred in four patients, three of whom had received peripherally administered magnesium chloride, and one patient reported transient ptosis after administration of magnesium sulphate 166 mmol over 18 hours in the setting of severe Irukandji syndrome. Magnesium sulphate administration appears to attenuate pain and hypertension in Irukandji syndrome and warrants further evaluation in this setting.