Caladiums (Caladium ×hortulanum) are popular ornamental plants widely grown for their bright colorful leaves. Pythium root rot, caused by Pythium myriotylum, is one of the few soil-borne diseases in caladium that dramatically reduces plant growth, aesthetic value, and tuber yield. Information on the reaction of caladium cultivars to P. myriotylum is not available, but would be valuable for integrated control of this disease and for breeding new resistant cultivars. Three Pythium isolates obtained from decaying roots of plants collected from a field production site and two greenhouses were evaluated for pathogenicity and potential use in experiments to screen commercial caladium cultivars for resistance. All three isolates were found to be highly virulent; they were able to cause obvious root rotting within 3 to 5 days and severe root rotting and leaf losses on susceptible cultivars within 10 days after inoculation. Nineteen major commercial cultivars were evaluated for their resistance to these isolates. Fifteen of the cultivars were susceptible or highly susceptible to Pythium infection. Four widely grown cultivars, `Candidum', `Candidum Jr.', `Frieda Hemple', and `White Christmas', were found to have a moderate level of resistance (partial resistance) to pythium root rot. Pythium infection also caused leaf discoloration, epinasty, wilting, and collapse. Regression analyses revealed a linear relationship between the root rot and leaf loss severity on Pythium-inoculated plants.