A list is presented of published reports of DNA polymorphisms found in the human genome by restriction enzyme analysis. While the list indicates the large number of restriction fragment length polymorphisms (RFLPs) detected to date, the information collated is insufficient to permit an estimate of heterozygosity for the genome as a whole. Data from our laboratory are therefore also presented on RFLPs detected using a random sample of cloned DNA segments. Such an analysis has permitted a first unbiassed estimate of heterozygosity for the human genome. Since this figure is an order of magnitude higher than previous estimates derived from protein data, the majority of polymorphic variation present in the human genome must, by implication, occur in noncoding sequences. In addition it was confirmed that enzymes containing the dinucleotide CpG in their recognition sequences detect more polymorphic variation than those that do not contain a CpG. Also presented are the clinical applications of DNA polymorphisms in the diagnosis of human genetic disease.