The advent of new antifungal drugs provides clinicians with therapeutic options, and it is anticipated requests for fungal susceptibility testing in vitro will increase. Our own laboratory's experience indicates that results can be provided promptly even to distant medical centers. The need in this setting is standardization of procedures, so correlations with in vivo outcome can be made with in vitro results. The variables affecting current testing methods are reviewed. Newer methods of testing are summarized, including our experiences using a spectrophotometer as a tool to assay inhibition. Our understanding of the mechanisms underlying the results with this and classical methods are presented. Several methods were applied to a population survey of Candida albicans isolates, using the drug flucytosine as an example. The results were correlated with in vivo outcome in a mouse model of systemic candidosis. The in vitro results, in general, predicted survival, but exceptions occurred, and it appears the in vitro results provide a relative but not absolute indication of outcome.