A rapid increase of female breast cancer has been reported in many areas of the world and the reasons are not fully understood. While some have attributed the increase to the increasing detection of early stage breast cancer through mammography screening, few studies have directly examined the time trend of in situ breast cancer specifically. This study included all incident cases of female breast carcinoma in situ reported to the Connecticut Tumor Registry between 1973 and 1992. The age-adjusted incidence rates and age-specific incidence rates were calculated by histology and by race. The age-adjusted incidence rates were standardised to the 1970 United States standard million population. The study found that the overall age-adjusted incidence rate of in situ breast cancer has increased dramatically, from 3.53/100000 in 1973-1975 to 17.51/100000 in 1991-1992. The increase was not uniform during the past two decades of cancer registration. In fact, most of the observed rise has occurred since the early 1980s. This increase was found in both Caucasians and Blacks. The results by histology indicate that the dramatic increase in carcinoma in situ is mostly attributable to an increase in ductal carcinoma in situ. The increase was also observed in all age groups 40 years and over. These results are consistent with the use pattern and the reported effect of mammography screening. Therefore, these results are qualitatively consistent with the idea that mammography screening is largely responsible for the recent upsurge in female breast cancer incidence in this population. The study also found that the age group 40-49 years in whites experienced a rapid increase in incidence that began in the same time period as the older age groups, but it has since levelled off. The potential impact of the highly publicized debate regarding the efficacy of mammography in this age group in recent years is discussed.