Much of the epidemiological heterogeneity of HD incidence reflects the behaviour of the NS subtype, at least in the USA. Incidence variation across races (except Asians) and time periods is most marked in this subtype. In young adults with HD, there is compelling evidence for social class modification of risk consistent with an infectious aetiology; limited data suggest that this effect occurs within the NS subtype, but considerable evidence indicates that it does not primarily involve EBV infection. Findings from familial aggregation studies and HLA associations point to inherited susceptibility to this subtype. Despite little sex difference for NS in young adulthood in the latest incidence data, parity nevertheless appears to be protective against this subtype for women. Therefore, the greater increase for females than males in the incidence of young-adult NS in recent years may reflect the impact of population trends towards later childbearing and lower parity. This change, as well as the concomitant smaller family sizes and growing affluence, could explain part of the burgeoning incidence of NS in young adults in the USA. These observations suggest that NS in young adults constitutes a separate disease, probably of infectious origin. The incongruous occurrence of this subtype in older adults, and the presence of EBV in some NS cases, could reflect heterogeneity within NS, for example, representing features of the cellular phase of NS (Cozen et al, 1992; Medeiros and Greiner, 1995). For the non-NS subtypes, many of the factors that predict risk of NS may also be relevant. Patterns of social class determinants in children and older adults, the age groups at risk for MC, support involvement of an infectious precursor given intense exposure, and EBV is a likely candidate, based on its high prevalence in these groups. However, little aetiological research has been directed explicitly at the non-NS subtypes. Considerable effort has gone into exploring an infectious aetiology of HD. Recently, this line of investigation has moved beyond social class determinants to molecular epidemiological studies of EBV and, to a lesser degree, other potentially involved viruses. The roles of genetic susceptibility and sex hormones also represent promising areas for exploration, particularly in their possible interaction with infectious agents and other environmental factors. Ultimately, clearer epidemiological understanding of HD will be aided by more precise classification of this disease at the molecular level.