Our aim was to compare udder health in groups of organically and conventionally managed cows, using data from a longitudinal study in a Swedish dairy-research farm. Management of the groups was identical except for feed composition and the feeding regimen. Our dataset included all lactating cows calving from 1 September 1990 to 31 August 2001 (145 organically and 151 conventionally managed cows). Udder health was assessed by the geometric average somatic-cell count (SCC) within 150 days after calving, by the number of monthly SCC tests >200,000 cells/ml within 150 days after calving and by presence of lactations with veterinary-treated cases of clinical mastitis. The effect of animal group was analysed by multivariable linear, Poisson and logistic-regression models, controlling for factors such as lactation number, breed, year, season and milk yield. The groups did not differ in any measure of udder health. We had power to rule out differences of at least 33,000 cells/ml in the geometric average somatic-cell count, an incidence rate ratio of 0.65 in the incidence of high-SCC milk-testing occasions, and an odds ratio of 0.43 in veterinary treated cases of mastitis.