A brief description of the Norwegian Health Card System for Cattle (NHCSC) is given. The incidence of clinical mastitis (CM) for dairy cows in Norway was determined using NHCSC data recorded from 1992 to 1995. Incidence density (ID) (which included repeated episodes of CM in the same cow) and various measures of risk were estimated. The ID for the entire study period, comprising almost 1.2 million cow-years, was 49 CM episodes per 100 cow-years at risk. Twenty-six percent of the cows with CM experienced at least two episodes of CM in 1 year. For the entire study period (1992-1995), the ID was 30 episodes per 100 cow-years at risk for acute CM (ACM). Annual risk of CM for fixed cohorts of cows that were at risk 1 January the particular year, as estimated by the actuarial method and accounting for the exact time of removal of culled cows, varied between 0.32 (1992) and 0.35 (1994 and 1995). Numerically similar risk estimates were found when using the density method for fixed cohorts. Lactational incidence risk for cows that calved in 1992-1994 varied between 0.32 and 0.34. In herds of a size greater than five cow-years, the crude ID of CM tended to decrease with increasing herd size. The ID of CM varied considerably between counties, and was higher in coastal areas than in inland areas.