The annual incidence of clinical mastitis was measured in 144 Holstein/Friesian dairy herds in England (average size 132 cows) during 1994, 1995 and 1996 by means of carefully defined mastitis indices. The mean annual incidence of the disease over the three-year period was 43.4 quarter-cases per 100 cows, and the disease affected 25.9 per cent of the cows in the herds, with 1.6 quarter-cases per affected cow. In terms of cow-cases, the mean incidence was 39.9 cases per 100 cows in the herd and hence the ratio of quarter-cases to cow-cases was on average 1:1. The proportion of repeat quarter-cases was on average 19.4 per cent, and the recurrence rate was 18.3 per cent. The new infection rate was 28.3 per cent.