Drriven by changes in agricultural production practices, nitrogen (N) inputs have increased steadily on Canadian farms. An agro-environmental indicator was developed to monitor potential water pollution by N: indicator risk of water contamination by nitrate-nitrogen (IROWC-N). The indicator links the residual soil nitrogen (RSN) indicator to climate and soil conditions to assess the likelihood of N moving through the soil and out of the agricultural system. The results are assessed in terms of Nlost via leached water (Nlost) and its concentration in the leached water (Nconc), with the IROWC-N risk classes based on Nlost and Nconc criteria. The estimated amount of Nlost in Canada ranged from 5.1 kg N ha-1 in 1991 to 6.4 kg N ha-1 in 2001. Nconc values remained fairly constant during the 1981 to 1996 census years (ranging from 3.7 to 4.5 mg N L-1), but increased sharply (27%) to 5.7 mg N L-1 in 2001 as compared with 1996. During the 1981 to 2001 period, close to 80% of the Canadian farmland area remained in the very low and low IROWC-N risk classes, but over the years 18% shifted to a higher risk class. In 2001, large areas (> 1 million ha) in the high risk IROWC-N class were found in Manitoba, southern and eastern Ontario and in Quebec. Provincial averages of Nlost over 5 census years (1981, 1986, 1991, 1996 and 2001) varied from less than 5 kg N ha-1 in Alberta and Saskatchewan to more than 20 kg N ha-1 in Ontario, Quebec and the Atlantic provinces. With the exception of Manitoba, provincial Nconc values did not exceed the Canadian drinking water guideline of 10 mg NO3-N L-1. In each of the census years, British Columbia, Alberta and Saskatchewan had more than 70% of the farmland area in the very low and low risk classes for IROWC-N. In Ontario and Quebec, most of the farmland area was either in the low or in the high risk class. More than 50% of the farmland area in New Brunswick, Nova Scotia and Newfoundland was in the very low, low and moderate risk classes, whereas in Manitoba and Prince Edward Island, more than 60% of the farmland was in the moderate and higher level risk classes for IROWC-N. Overall, the 20-yr trend in risk of water contamination by N was worsening. Key words: Water contamination by nitrogen, nitrate, water quality, Soil Landscapes of Canada, Census of Agriculture