A detailed study of the epidemiology of subclinical nematode infections in adult dairy cows was conducted on five well-managed English dairy farms during the 2002 grazing season. These same farms had also participated in a similar study in 1978-1979 and thus provided a unique opportunity to compare the epidemiological findings after an interval of over two decades. Common factors, including the prevalence of infection, faecal worm egg output, pasture larval count and nematode genera present, and estimated daily larval intake at pasture, were compared between the two studies. Subclinical roundworm burdens, as judged by faecal egg counts, were widespread in dairy cows in 2002, but the prevalence of animals with patent infections and the magnitude of the worm egg output were significantly lower than in 1978-1979. Both the prevalence of infection and mean faecal worm egg output exhibited a marked seasonal peak during the summer months in 2002. Pasture larval numbers were, however, nearly three times higher in 2002 with Ostertagia dominating the nematode genera to which cows were exposed at pasture in both surveys. Factors considered likely to account for differences in parasite epidemiology include the marked increase in herd productivity over the intervening period, the increased proportion of Holstein genetics with the consequent increase in milk yield and feed intake, changes in grassland management and increased stocking rate seen on the majority of farms.