Serum arabinitol concentrations and arabinitol/creatinine ratios were determined in 25 cancer patients with invasive candidiasis, 73 severely ill cancer patients who did not have invasive candidiasis, and 15 uninfected patients with chronic renal failure. Elevated arabinitol concentrations were found in patients with renal failure and invasive candidiasis. Serum arabinitol concentrations exceeded 1.19 micrograms/ml in 13 of 18 patients with invasive candidiasis who were studied when renal function was normal and in three of 52 control patients. Among patients with renal failure, 10 of 14 with invasive candidiasis but only four of 36 control patients had serum arabinitol concentrations of greater than 5.85 micrograms/ml. Serum arabinitol and creatinine concentrations were strongly correlated. The arabinitol/ creatinine ratio exceeded 1.5 in 16 of 25 patients with invasive candidiasis but in only three of 88 control patients. Increased serum arabinitol concentrations appear to reflect increased production of arabinitol by yeast in individuals with invasive candidiasis.