Several recent studies have investigated the effect of shortened dry periods on milk production in the subsequent lactation. What is lacking from these studies is an understanding of the effect that a shortened dry period has on udder health. Four herds, 156 cows, were studied to determine if a shortened dry period (30 d) had a negative effect on mammary gland health during the subsequent lactation as opposed to cows assigned to a long, 45 or 60 d, dry period. Cows in 2 herds were assigned to either 30- or 60-d dry periods (group I), whereas cows in the other 2 herds were assigned to either 30- or 45-d dry periods (group II). Intramammary instillation of commercial preparations of cephapirin benzathine, 300 mg (dry cow formulation), was given to cows assigned a 45- or 60-d dry period length protocol, and 200 mg (lactating cow formulation) was administered to cows assigned a 30-d dry period. Differences in response variables to dry period length were compared within group. Cure rates for 60- vs. 30-d dry period cows were 72% (28/39) vs. 81% (30/37) and 74% (25/34) and 73% (27/37) for 45- vs. 30-d dry periods. Differences were not statistically significant for either comparison group. The majority of intramammary infections were caused by the minor pathogens, coagulase-negative staphylococci (n = 102) or Corynebacterium bovis (n = 11). Only 11 cows had intramammary infections by major pathogens. The herd average percentage of new intramammary infections ranged from 6 to 9% and did not differ among herds between treatment groups. Linear somatic cell counts were not significantly affected by dry period length during the first 6 to 7 mo of the subsequent lactation. Milk production did differ between groups. Mature equivalent milk production was greater in group I cows given a 60-d dry period (11,942 +/- 2,059 kg) as opposed to those given a 30-d dry period (10,749 +/- 2,321 kg). Cows given a 45-d dry period did not produce more milk than cows with a 30-d dry period in group II. Although shortening the dry period to 30 d did not have untoward effects on mammary gland health as measured by intramammary infections or milk somatic cell counts, production may be adversely affected when dry periods are shortened to 30 d.