The Canadian Council on Animal Care (CCAC) publishes data on over 25 species of animals used in science, and the US Department of Agriculture publishes data on six of those species. Between 1980 and 1999, the reduction in animal use was found to be correlated between Canada and the USA for dogs (r = 0.944, p < 0.001), cats (r = 0.839, p < 0.001), rabbits (r = 0.852, p < 0.001) and hamsters (r = 0.716, p < 0.01), with no significant correlation found for non-human primates and guinea-pigs. On the basis of the four species where correlation between the two countries was found for reduction in use, the mean ratio of the number of animals used in the USA compared to the number used in Canada was 17.0 +/- 7.5. The CCAC data for these six US-regulated species were used in an analysis of regression with multiple predictors to test whether they could be used to predict the total number of animals used. No significant correlation was found. However, using the same analysis, rats, mice, fish and birds were found to be highly correlated with the total number of animals used (r2 = 0.9835, p < 0.005). The regression equation developed by using Canadian data was validated using UK animal use numbers. An almost perfect fit between the estimated values provided the evidence that total animal use in Canada and the UK decreased at about the same pace during the 1990s. Animal use data can be a useful tool to monitor the implementation of reduction measures. However, their use for the monitoring of refinement measures requires care and analysis. For example, the sustained downward trend in the number of experiments causing severe pain in unanaesthetised animals (category of invasiveness [CI] E) observed in Canada and the USA between 1996 and 1999 is indicative of effective refinement, but it would be misleading to interpret the increase in the number of animals used in Canada under CI D in 1997 as an indication of greater pain and distress. In fact, the larger number of animals in CI D resulted at least in part from the implementation of new CCAC guidelines designed to ensure better monitoring of transgenic animal care and use.