The aim of the presented mini-review is to review the literature data referring to opportunistic mycoses in pet dogs and cats suffering from other concurrent diseases, comparable to human medical disorders with high risk of secondary mycoses. This review also presents the preliminary results of a project aimed at understanding the fungal colonization and occurrence of secondary mycoses in pets suffering from metabolic disorders, neoplasms and viral infections. The incidence of opportunistic mycoses is higher in such individuals, mostly because of their impaired immunity. The main risk factors are primary and secondary types of immunodeficiency connected with anti-cancer treatment or neoplastic disease itself. Moreover, literature data and the results of our investigations show that Candida yeasts are prevalent among diabetic animals and indicate that these fungi are the main etiological agents of secondary infections of the oral cavity, GI and urogenital tracts. Other important conditions possibly favoring the development of mycoses are concurrent infections of cats with FeLV and FIV viruses. Thus, in all cases of the mentioned underlying diseases, animals should be carefully monitored by repeated mycological examination, together with inspection of other parameters. Also, the prophylaxis of opportunistic mycoses should be carefully considered alike other factors influencing the prognosis and the outcome of primary diseases.