To quantitatively assess the role of Candida species in antibiotic-associated diarrhea (AAD), stool samples from a total of 395 patients and control subjects were cultured in differential isolation medium: 98 patients had AAD, 93 patients were taking antibiotics but did not have diarrhea (A(+)D(-)), 97 patients were not taking antibiotics but had diarrhea (A(-)D(+)), and 107 patients were control subjects (A(-)D(-)). In addition, secreted aspartyl proteinase (Sap) production was tested. In AAD patients, Candida positivity (77/98) and Candida overgrowth (62/98) were not different from that among A(+)D(-) patients (75/93 [P= .860] and 52/93 [P= .375], respectively). Candida overgrowth among A(-)D(+) patients (40/97, P= .003) was less frequent than among AAD patients, but Candida positivity was not different (80/97, P= .612). In control subjects, Candida positivity and overgrowth were less common than in all other groups. Production of Sap did not differ between patients with AAD and control subjects (P= .568 and P= .590, respectively). Data indicate that elevated Candida counts are a result of antibiotic treatment or diarrhea rather than a cause of AAD.