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The numbers of dead cattle in 1977/1978-1985/1986, expressed as a percentage of the total cattle numbers, for six regions in Botswana and for the country as a whole, were analysed as a function of a simple 10-day water-balance model which contains only estimated potential evapotranspiration, rainfall, one single rangeland-cattle factor and one maximum available soil moisture reserve as input parameters. The output of the model is a livestock performance index, which expresses (as a percentage) the extent to which the assumed water requirements of cattle and range were satisfied by rainfall. It was found that, depending upon the area, between 75% and 90% of the variability of the cattle death ratios is accounted for by the index calculated for the past, the second last and part of the third last season. On a national scale, 93% of the interannual variability of cattle death ratios and 93% of the variability of the percent departure of cattle numbers from the normal cattle population growth curve is accounted for by the livestock performance index calculated for the above-mentioned period. Further analysis of the results obtained for each area and tests of accuracy and precision of the model show that the index can be used as a qualitative indicator of rainy season qualities for livestock performance on an areal scale and as a quantitative indicator for the assessment of cattle death ratios and numbers on a national scale.