An approach to the expediting of the diagnosis of opportunistic systemic mycoses is presented. Communication between clinician and microbiologist is basic to this approach. The importance of the clinical assessment of the individual patient, coupled with a high index of suspicion, is stressed. Our experience with 11 of 42 cases of systemic mycosis over a 28-month period is analysed. For the diagnosis of fungaemia a method for the microscopical examination of peripheral blood is briefly evaluated, and a membrane filter blood culture technique is shown to be valuable, yielding results in 16-24 hours. In the absence of fungaemia the considered microscopical examination of suitable specimens, when feasible, is the most rapid method available. Serological methods may be helpful in early diagnosis, but this is often hampered by the absence of baseline sera and by the lengthy nature of some tests. Newer indirect methods such as gas chromatography are being developed but have not yet been used routinely.