A patient developed root surface caries and loss of teeth following the jejunoileal (JI) bypass. We attempted to confirm the association of root surface caries with the JI bypass and explore the mechanisms by which it occurs.The number of root surface caries per year after, and an equal period of time before, a JI bypass was determined in 18 patients. These 18 patients and 5 normal controls gave stimulated saliva samples for measurement of chloride, bicarbonate and pH. 4 JI bypass patients and 4 normal controls gave timed stimulated saliva samples for measurements of volume.7 of 18 JI bypass patients had >0.5 root surface caries per year after the operation but none before (p 12 meq/l in 3 of the 18 JI bypass patients but in none of 5 controls (p<.05). The salivary pH and bicarbonate were 6.38+/-0.48 vs 6.92+/-0.21 and 2.81+/-2.1 meq/l vs 5.8+/-1.2 meq/l in the JI bypass group and the control group respectively (p<.05). The stimulated saliva was 2.3+/-1.2 vs 4.5+/-1.4 cc/min in the JI bypass group and control group, respectively (p<.02).Root surface caries are more frequent after JI bypass. This may be due to decreased saliva flow and a reduced salivary buffering capacity.