Advances in molecular biology have had a tremendous impact on our understanding of the pathogenesis of hereditary disorders, tumours and infectious diseases. It is anticipated that recombinant DNA technology will gradually assume an important role as a diagnostic tool in medicine, since at least some of the techniques are now ready for routine use in the clinical laboratory. The most lucrative application (and hence the most competitive market) for DNA probes will be the detection of bacteria, viruses and other microbiological organisms by nucleic acid hybridization techniques. The value of recombinant DNA technology for prenatal diagnosis and carrier detection in genetic disorders is now firmly established. Analysis of DNA and RNA obtained from tumours may provide diagnostic information of practical relevance in carefully selected cases. It is, however, unlikely to challenge the established value of the more "traditional" diagnostic tools such as histopathology and immunophenotyping.