The incidence of clinical mastitis and distribution of pathogens in dairy cows was estimated in 171 randomly selected dairy herds in the southern Netherlands. A total of 1103 quarter cases were reported. The average annual incidence rate was 12.7 quarter cases per 100 cows per year. The most frequent isolates from clinical cases were Escherichia coli (16.9 per cent), Staphylococcus aureus (14.4 per cent), Streptococcus uberis (11.9 per cent) and Streptococcus dysgalactiae (8-9 per cent). Most cases were reported in early lactation: 25.4 per cent in the first month of lactation for all cows, and 39.1 per cent in the first month for first lactation cows. The rear quarters had a significantly higher incidence rate than the front quarters. Cows with an E coli infection showed more general clinical signs than cows infected with S aureus, S uberis and S dysgalactiae . A significantly higher incidence was observed in herds with a low (<150,000 cells/ml) bulk milk somatic cell count than in herds with a count above 250,000 cells/ml.