It is generally assumed that a decreased salivary secretion rate will promote plaque accumulation and increase the risk for caries, gingival inflammation, and mucosal infections. In this study, the effect of hyposalivation on the oral microflora was examined. The following micro-organisms were analyzed in rinsing samples from 14 subjects with hyposalivation: the total number of anaerobically growing micro-organisms, alpha-hemolytic streptococci, mutans streptococci, lactobacilli, Fusobacterium nucleatum, Prevotella intermedia/Prevotella nigrescens, Staphylococcus aureus, Candida albicans, and enterics. The study group, age 53 +/- 7 years, had no history of radiation therapy and showed no signs of inflammation in their salivary glands on biopsy. All were dentate with a mean of 24 +/- 3 teeth. Their salivary secretion rates were 0.03 +/- 0.02 mL/min (unstimulated) and 0.84 +/- 0.65 mL/min (stimulated). The control group was matched to the hyposalivation group according to age, sex, and number of teeth. There was a significantly increased number of lactobacilli, and a tendency, not statistically significant and with large variations within the groups, toward a higher proportion of mutans streptococci and a lower proportion of alpha-hemolytic streptococci in the hyposalivation group. The presence of micro-organisms associated with gingival inflammation and mucosal infections was comparable with that in the healthy controls. The results indicated that a low salivary secretion rate mainly promotes a flora associated with the development of caries.