The present study reports about the effect of doxycycline and/or metronidazole on colonization by Candida organisms of the human gastrointestinal (GI), oropharyngeal tract and vagina. Treatment with doxycycline or metronidazole for 10 days increased, but not significantly, the GI, oropharyngeal or vaginal colonization by Candida species. The combination of doxycycline and metronidazole, used for the same period, caused a significant increase of 2.5 log10 CFU/g of stools (mean) colonization by Candida. Likewise, 2 out of 9 patients treated with the combination had substantially increased colonization of their vagina by Candida species. This effect, however, could not be expressed statistically due to the semiquantitative nature of the vaginal cultures. In contrast, the combination did not increase oropharyngeal colonization. In conclusion, doxycycline and metronidazole as monotherapies, did not increase significantly Candida colonization in the cavities examined. The combination of doxycycline and metronidazole had a substantial effect, increasing the GI and vaginal colonization by Candida organisms.