Epidemiological studies of genetic differences in disease susceptibility often estimate the relative risks (RR) of different genotypes. Here I provide an approach to calculate the relative fitnesses of different genotypes based on RR data so that population genetic approaches may be utilized with these data. Using recent RR data on human haemoglobin β genotypes from Burkina Faso, this approach is used to predict changes in the frequency of the haemoglobin sickle-cell S and C alleles. Overall, it generally appears that allele C will quickly replace the S allele in malarial environments. Explicit population genetic predictions suggest that this replacement may occur within the next 50 generations in Burkina Faso.