The occurrence of yeasts in 967 microbiological endodontic samples taken from root canals in persistent endodontic infections was studied. The sampling was done by general practitioners in various parts of Finland from root canal infections which did not respond favourably to standard conservative therapy. The samples were cultivated aerobically on a non-selective enriched horse blood agar medium, on TSBV agar medium in 5% CO2 and anaerobically on horse blood agar medium. Micro-organisms were found in 692 of the samples while 275 showed no growth. Forty-eight fungi were isolated from 47 samples which is 7% of the culture-positive samples. Twenty yeast strains were identified further by their colony morphology, growth and cellular characteristics and patterns of carbohydrate assimilation. All isolates except one belonged to the genus Candida. Candida albicans was the most common species. C. glabrata was found together with C. albicans in one sample. C. guilliermondii, C. inconspicua and Geotrichum candidum were each isolated once. Yeasts were found in pure culture in six samples and together with bacteria in 41 samples. In all the samples except two, the accompanying facultative bacteria were Gram positive. The most frequent of them were alpha- and non-haemolytic Streptococcus species which were found in 31 samples. Anaerobic bacteria were isolated together with yeasts from 12 root canals. They included both Gram positive species such as Peptostreptococcus micros and Gram negative species such as Fusobacterium nucleatum. The regular isolation of yeasts, also in pure culture, indicates that yeasts may have an important role in cases of apical periodontitis persisting after conventional treatment.