Pneumonitis is the most serious complication of varicella zoster infection and may be fatal despite antiviral therapy and assisted ventilation. We report two cases of varicella pneumonitis which were complicated by severe pneumonitis. Both patients deteriorated on maximal ventilatory support and were then given extra-corporeal membrane oxygenation (ECMO) and recovered. One patient subsequently developed lymphoma and the other was found to have an immunoglobulin subclass deficiency. ECMO should be considered for patients with varicella pneumonitis who do not respond to conventional treatment. Those with severe pneumonitis should be investigated for underlying immunodeficiency.