Tobacco smoking is a well-recognized risk factor for Legionnaires disease. However, it may be potentiated by cannabis use, as there is strong evidence that Δ(9)-tetrahydrocannabinol impairs immune functions in vitro and in vivo. We report herein two out of three cases of severe Legionnaires disease in three men with no overt comorbid illnesses, aged 38, 28, and 48 years, respectively. All of them were heavy cigarette and cannabis smokers.