Twelve trials designed to test the effect of monensin on milk production were carried out at eight different research farms. Data from these studies were evaluated for effects of monensin on health and reproduction. Monensin was added to the concentrate starting either 2 wk before (293 cows) or 5 wk after calving (601 cows) for periods ranging 16 to 37 wk. Applied after calving, the incidence of clinical mastitis was reduced in the monensin-treated animals. Monensin decreased the rate of intramammary infection (approximated by a change from below to above 250,000 somatic cells in milk) in first lactation heifers by 13%. Indicated by an adjusted incidence rate ratio of 0.58 (P = 0.03), the case incidence of noninfectious lameness decreased from 31% in control cows to 18% in cows receiving monensin before calving. The time from calving to first service was shorter for cows fed monensin before calving. Monensin reduced calving to conception intervals in cows with endometritis during the pretreatment period. No significant effects of monensin were observed for the duration of intra mammory infection (somatic cells in milk above 250,000), infectious lameness, endometritis, cystic ovarian disease, a summary category of 'other diseases,' times from calving to first observed estrus, and from first service to conception.