Four different field studies including 64, 140, 850 and 180 herds were used to investigate the association between pulsator characteristics and udder health. The results indicate that the best udder health is found in herds with a d phase > 330 ms and a pulsation rate > 55 min-1. A d phase of < 250 ms was associated with significant elevation in bulk milk somatic cell count, a higher incidence of acute clinical mastitis and a higher frequency of cows having mastitis associated with major pathogens (mainly Staphylococcus aureus) as compared with a d phase of > 331 ms. There was also a significant positive association between a shorter d phase and a high incidence of teat lesions needing veterinary treatment. Herds having no such teat lesions had pulsators with significantly longer d phase than herds with teat lesions (300 ms v. 288 ms). These results strongly suggest that in high-line milking machines the d phase should not be as short as 250 ms and should preferably be approximately 300 ms, and the pulsation rate should be at least > 55 cycles min-1. There was a strong relationship between the effect of d phase and the rate of pulsation.