The annual frequency of hysterectomy was monitored in the Canton Ticino, Switzerland, from 1977 to 1986. From February to October, 1984, there was a public information campaign in the mass media about rates of and need for hysterectomy. After the start of the campaign and during the following year the annual rate of operations per 100,000 women of all ages dropped by 25.8%, whereas in the reference area (Canton Bern), where no information was given to the public, hysterectomy rates increased by 1%. In the same period the hysterectomy rate per 100,000 women aged 35-49 declined by 33.2%, and the number of hysterectomies performed annually per gynaecologist decreased by 33.3%. In Canton Bern these rates were unchanged. The decline began 2 months after the start of the information campaign. The reduction in the number of hysterectomies was greater (p less than 0.001) in non-teaching hospitals (31.9%) than in teaching hospitals (18.1%). Information on regional rates and on the need for hysterectomy given through the mass media to the general population can change professional practices.