We studied the extent of patients' utilization of complementary medicine (CM), and their knowledge and attitude regarding it in 740 patients visiting 25 randomly family medicine practices. One-third reported using some kind of CM, usually a natural drug (67.6%), often together with conventional drugs. Among the users of natural drugs, 56.2% believed they caused no side effects, 44.7% never reported natural drug usage to their physician, and 11% did so only rarely. There was a significant correlation (P = 0.03) between the belief that natural drugs can cause adverse effects and the tendency to report their usage to the family physician. Compared to nonusers, the typical user of any kind of CM was older (with a 1.05-fold increase for every year of age), defined his/her health status as bad (8.6-fold higher incidence), visited the family doctor more often, and was of Eastern European origin. In conclusion, although the use of natural drugs is extensive, patients' knowledge of their potential adverse effects is poor. A public educational campaign, with inclusion of the need to report such usage to the family physician, should be implemented, and questions on the use of complementary medicine/natural drugs should be incorporated as an integral part of the history taking by primary care physicians.