Disease is an integral element of agricultural and natural systems, but the roles pathogens play in determining ecosystem response to elevated CO2 have rarely been examined. To investigate whether disease can alter the response of plants to CO2, we examined the effects of doubled CO2 (∼700 μmol mol−1) on Avena sativa infected with barley yellow dwarf virus (BYDV), a common pathogen of cereals and grasses. Oats infected with BYDV showed a significantly greater biomass response to CO2 enrichment than did healthy plants. Root mass of diseased plants increased by 37–60% with CO2 enrichment, but was largely unaffected in healthy plants. CO2 enrichment increased midday leaf-level photosynthesis and instantaneous water use efficiency by 34 and 93% in healthy plants and by 48 and 174% in infected plants. Foliar carbohydrates increased with both CO2 enrichment and BYDV infection, but the two factors affected individual pools dissimilarly. CO2 enrichment may alter the epidemiology of BYDV by increasing the persistence of infected plants.