The objective was to evaluate the relationship between dietary and serum phosphorus (P) during the transition period and its relationship to fertility measures. Blood samples were collected into vacuum tubes at the morning feeding from cows (n = 4129) in 8 Holstein (H) herds in the summer (S) and winter (W), as well as 8 Jersey (J) herds in the S. Samples were placed on ice immediately. After processing, samples were stored at −20°C until analysis. On the day of sampling, total mixed ration samples were collected for subsequent analysis. Associations were tested by logistic regression, correlation analysis, and ANOVA. During S, P in the ration (% DM) averaged 0.35 ± 0.05, prepartum H; 0.45 ± 0.12, prepartum J; 0.42 ± 0.03, postpartum H; and 0.47 ± 0.07, postpartum J; and during W averaged 0.35 ± 0.05, prepartum H and 0.42 ± 0.03, postpartum H. Serum P values (mg/dl) for wk −3, −2, −1, 1, 2, and 3 relative to calving were 5.23 ± 0.09, 5.26 ± 0.19, 5.34 ± 0.10, 4.79 ± 0.12, 4.97 ± 0.12, and 5.40 ± 0.10 for S-H; 6.38 ± 0.09, 6.30 ± 0.07, 6.21 ± 0.07, 5.88 ± 0.09, 5.78 ± 0.08, 5.65 ± 0.10 for S-J; and 5.23 ± 0.12, 5.29 ± 0.20, 5.71 ± 0.1, 5.08 ± 0.13, 5.44 ± 0.11, and 5.53 ± 0.10 for W-H. The ration and serum P correlation coefficients (P < 0.0001) ranged from 0.14 over all weeks to a high of 0.34 for wk 3. During S, serum P concentrations were significantly associated with breed for wk −3, −2, −1, 1 and 2 (P < 0.0001), while lactation was significant for wk −1, wk 2, and wk 3 (P = 0.0157) and tended to be significant during wk −2 (P < 0.0575). In univariable analysis, P serum concentration appeared to be associated with days to first service (DFS) and days open (DO); however when season and dairy were added in multivariate analysis, P serum concentration was no longer significant (P > 0.10). When estimating the odds of pregnancy in either the first 90 (P90) or 150 (P150) d postpartum, serum P concentration was not significant in the multivariate analysis; when season and lactation were included. Serum P concentrations in the peripartum period were correlated with P intake; however, the main factors influencing the reproductive factors of DFS, DO, P90, and P150 were breed, season and lactation rather than serum P concentration.