The effects of milking cows with two different liners were measured for a period of 8 months with 115 Danish Holstein cows divided into two groups. Group H and L animals were milked with liners with mouthpiece cavity heights of 30 and 18 mm respectively (other dimensions also differed between the two liners). Average teat lengths of first lactation cows were 45 and 40 mm for front and rear teats. Older cows had teats approximately 10 mm longer. There was no difference in milk yield or milk flow rates between the two groups. Average machine-on time was shorter for group L, and first lactation cows of group L were less restive. The frequency of red and blue discoloured teats immediately after milking was higher for group H, and teat length increased on average 5 mm during lactation with no increase for group L. The small overall differences in udder health between the two groups were not significant. Udder health was better for first lactation cows of group L, even though liners of group L slipped more often and cows with recorded liner slip had poorer udder health. We conclude that special attention should be given to first lactation cows when liner type is selected for a herd. We propose that breeding programmes should ensure that teat length is kept above 50 mm.