Viruses have long been considered a major environmental factor in the aetiology of Type I (insulin-dependent) diabetes mellitus and recent work has greatly confirmed and extended this role. In addition to the enteroviruses, there are several other viruses which, from time to time, have been considered potential causal agents for human diabetes. With the exception of rubella, their role is not clear. The relation of enteroviruses with Type I diabetes has only been properly clarified by the use of new technologies, especially those based on polymerase chain reaction methods to identify them in blood. It is now evident from studies in several countries that enterovirus infection accompanies or precedes the onset of diabetes in many children. It is less certain whether this is true for older persons or for other types of diabetes. Enterovirus infection in pregnancy has also been suggested to cause diabetes in children. The infection with enteroviruses seems to be linked to the induction of islet-cell autoantibodies as well as to the expression of interferon-alpha. Both of these events are connected with islet-cell destruction. It has become increasingly important to establish the nature of the infecting virus in the early stages of diabetes. It seems likely that a number of viruses of the coxsackie or echovirus type are involved, although the nature of the nucleotide sequences responsible for diabetogenicity remains elusive.