Familial hypercholesterolaemia (FH) is one of the most commonly inherited diseases. It is characterised by an abnormal LDL receptor resulting in a selective elevation of serum LDL and cholesterol levels. The correlation between FH and premature heart disease means that these patients contribute significantly to the number of individuals presenting with coronary heart disease. In the work described here cDNA probes to LDL-receptor were used to assess the usefulness of recombinant DNA technology to diagnose familial hypercholesterolaemia. A 3' probe to the LDL-receptor which detects a restriction fragment length polymorphism (RFLP) in linkage disequilibrium with normal and mutant LDL-receptor genes, was found to be potentially informative in 20% of the families studied. In addition a 5' probe to the LDL-receptor may be capable of directly detecting mutations in some 6% of families. We suggest that until further work has established other RFLP's or oligonucleotide probes are synthesised to directly detect mutant LDL-receptor genes, recombinant DNA technology is only of limited value for diagnosing familial hypercholesterolaemia.