Mastitis monitoring and control programs were instituted in 4 Illinois dairy herds for 12 months. Two herds had high mean monthly bulk tank somatic cell counts (> 490,000 cells/ml) and 2 had low mean monthly bulk tank somatic cell counts (< 260,000 cells/ml) at the start of the study. The mastitis monitoring and control programs included mandatory mastitis control measures, as well as individualized control measures that were based on results of bacterial cultures of milk, bulk tank milk analyses, milking machine and milking procedure evaluations, and environmental inspections in each herd. Changes in mastitis prevalence, clinical mastitis incidence, milk yield, and individual cow somatic cell counts were evaluated, and an economic analysis was performed for each herd. Mastitis-associated economic losses during the study period ranged from $161.79 to $344.16/lactating cow in the 4 herds. Gross economic benefits resulted when mastitis-associated losses were lower with the monitoring and control program than predicted without it. There were no gross economic benefits in the herds with low somatic cell counts, and, when the marginal costs of the programs were added, there were large net losses ($84.06 and $113.01/lactating cow) in those herds. Gross economic benefits resulted in both of the herds with high somatic cell counts. However, in 1 of the herds, the marginal costs of the program exceeded the benefits, resulting in a net loss of $12.96/lactating cow. The net loss was attributed primarily to poor producer compliance with recommendations. There was a net economic benefit of $19.11/lactating cow in the other herd with high somatic cell counts, in which producer compliance was better.(ABSTRACT TRUNCATED AT 250 WORDS)