The purpose of this study was to investigate oral environmental risk factors involved in caries incidence in the elderly. The subjects were 91 elderly subjects with fixed prostheses and 65 who wore removable partial dentures. We examined oral environmental factors by medical interview, clinical examination and saliva tests. The following 11 oral environmental factors were judged to be related to caries incidence: number of medicines taken daily, frequency of meals, frequency of brushing, amount of plaque on natural teeth, stimulated and unstimulated salivary flow rate, stimulated and unstimulated salivary buffer capacity, and salivary microbial counts of Streptococcus mutans (SM), Lacto-bacilli (LB) and Candida (CA). We compared these factors for subjects with fixed prostheses (the fixed group) and those wearing removable partial dentures (the denture group). Significant differences were observed between the two groups in plaque adhesion on the natural teeth and salivary microbial counts of SM, LB and CA. These finding demonstrate the importance of carefully selecting a prosthetic appliance for elderly by taking into consideration of oral hygienic conditions and salivary microbial counts. We found that fixed prostheses are less cariogenic. (J Osaka Dent Univ 2003; 37: 109-114.)