The objective of this study is to identify gender-related differences in the types of symptoms and drugs reported to cause an adverse drug reaction. Patient data from the Sunnybrook Health Science Centre ADR Clinic for the period from April 1986 to May 1996 were reviewed. Of the 2,367 patients assessed, 74.1% were female. The mean age of the patients was 43 +/- 17 years. Drug classes most frequently reported to elicit an adverse event were general antiinfectives (60.4%), nervous system agents (21.5%), and musculoskeletal agents (3.7%). Skin-related reactions accounted for 49.0% of all reported adverse drug reactions. More than one agent was reported to be responsible for the adverse drug reaction(s) in 50% of the female patients, versus 33.1% of all male patients. Of the female patients, 47.6% were referred for skin or oral challenge testing, versus 41.6% of the male patients. Of the female patients, 6.2% tested positive to the agent compared with 6.1% of all male patients. These results support previous findings that female gender is a risk factor for the development of adverse drug reactions. Further work is required to elucidate the mechanisms explaining the differences observed between male and female patients.