The performance of lactating Holstein cows in response to P supplementation was determined in a 2-yr study. Each year included confinement feeding for approximately the first two-thirds of lactation and grazing for the remaining one-third of lactation. In yr 1, 42 cows were assigned at calving to a low or high P diet within parity. Fourteen cows from the low P group and 16 cows from the high P group continued with their treatments for a second year. Also in the second year, 12 new cows were included in the low P group and 11 in the high P group. Thus, a total of 95 lactations with 65 cows were used in the trial, and 30 of the cows were used in both years. The dietary P was 0.38 and 0.48% during confinement feeding and approximately 0.31 and 0.44% during grazing for the low and high P treatments (dry basis). When all cows were used to obtain treatment means, milk yield for 308 d of lactation was 9131 and 8860 kg in yr 1, and 9864 and 9898 kg in yr 2 for the low P and high P groups, respectively. Blood serum inorganic P tended to be slightly lower for the low P than for the high P group during most of lactation; all concentrations (5.6 to 7.4 mg/dl) were within normal ranges. Reproductive measures were similar between groups in both years. When just the cows completing two lactations (N = 30) were evaluated, milk yield was 9072 and 8780 kg in yr 1 and 11,457 and 11,358 kg in yr 2 for the low P and high P treatments, respectively. Reducing dietary P from 0.48 to 0.38% for 2 yr did not impair milk production or reproductive performance.