The clinical efficacy of assays for Candida albicans antigens by latex agglutination and for antibodies by indirect haemagglutination were prospectively evaluated in the diagnosis of invasive Candida infections in 38 children suffering from acute leukaemia or other malignant disease. The controls were 74 other patients without any malignancy; 72 of these had no signs or symptoms of fungal infections, but 2 had an invasive C. albicans infection. During a period of 21 months, 302 serum samples were tested by both assays, and the results were compared with clinical and other microbiological data. Invasive fungal infection was diagnosed on clinical grounds in 2 of the immunocompromised children, and periodic gut colonization was demonstrated in 11 of 36 (31%) children in this group. Positive Candida antigen was detected in 14 patients (37%) and a positive antibody titre in 7 patients (18%). Colonization was not correlated with antigen or antibody titre. Compared with the presence of invasive fungal infection, the antibody assay detected all four infections, whereas the antigen assay detected one of the two C. albicans septicaemias. Although the Candida antibody assay performed well, a detectable change in antibody titres appeared only slowly. Thus it was of no clinical help when antifungal treatment was to be considered. Follow-up of antibody titres, however, gave confirmation of the presence of fungal infection as well as the response to antifungal treatment.