Surgically treated patients with, head and neck cancer commonly receive irradiation as an additional therapy. During irradiation therapy, patients experience changes in the occurrence of candidiasis and a change of isolated yeast-like species compared to findings before and three weeks after finalization of irradiation therapy. Out of 25 tested patients in this study, only 20% suffered from candidiasis with two species of candidae (C. albicans and C. glabrata) before treatment and three weeks after the finalization of the tratment (C. albicans and C. krusei), and 36% of them suffered from candidiasis and five different yeast-like species were isolated (including four different Candida species) during the treatment. Number of patients with candidiasis, number of positive oral smears, the same as number of yeast like species isolated were all significantly increased during the irradiation therapy (p≤0,05). There was 30% less positive oral smears on yeast like species before and three weeks after irradiation treatment in this group of patients with head and neck cancer than 50% normaly expected in healthy population. It is significant that all the patients with a positive oral smear on yeast-like species also suffered from candidiasis before and after irradiation therapy, hence among that patients candida was not saprophyt but exclusively pathogen. So, we can conclude that even during the irradiation therapy candida continued to be a pathogen regardless of mucositis also present at the time.