Time trends in incidence of in situ carcinoma of the breast over the period of 1977-1994 were analysed by histological type and age group, using data from the Cancer Registry of the Swiss Canton of Vaud, covering a population of approximately 295,000 women in 1990. Overall, 399 women with carcinoma in situ of the breast were registered. Age-adjusted (on the world population) incidence rates increased from 2.1/100,000 women in 1977-1979 to 9.4 in 1992-1994 (4.5-fold increase), and the rise was appreciably greater for ductal carcinoma in situ (DCIS, from 1.0 to 7.1/100,000) than for lobular carcinoma in situ (LCIS, from 1.0 to 2.3/100,000). The largest increase was registered between 1983 and 1992, whereas incidence rates tended to level off over the most recent calendar period. The upward trends were observed in all age groups above 40 years, but chiefly in the age groups 40-49 and 50-69, whose rates were higher than for women over the age of 70 years. These findings suggest that the adoption of mammography in this population during the 1980s is partly or largely responsible for the observed trends. Incidence rates of in situ breast carcinoma in Vaud are still significantly lower than those observed in the United States (between 15 and 17/100000) over the same calendar period. This is attributable to a lower utilisation of mammography, but may also be related to real differences in the incidence of breast lesions in Europe compared to North America.