Data from general health surveys completed by random samples of adult members of a large Northern California health maintenance organization in 1996 and 1999 were used to investigate (a) the prevalence of use of 15 complementary and alternative medicine (CAM) modalities by adult members in 1999, (b) how prevalence varied by age group and gender, and (c) which modalities were increasing in popularity. While Northern California is not representative of the nation as a whole, it tends to be a harbinger of trends to come. The most widely used modalities in 1999 were herbal and other nonvitamin/nonmineral nutritional supplements, prayer/spiritual healing done by oneself, chiropractic, massage therapy, and mind-body medicine modalities. However, use of the different modalities varied significantly by age and gender, and rates of use of nutritional supplements and the manipulatives were approximately doubled, when restricted to subpopulations at high risk, for use because of relevant health conditions. There appeared to be a statistically significant, but modest increase in CAM use, mostly due to a large increase in use of nutritional supplements. Implications for how CAM use should be tracked for ambulatory care populations and the importance of addressing nutritional supplement use in ambulatory clinical care are discussed.