The main objective of this study was to investigate factors that could influence the success of a resynchronization protocol with and without progesterone supplementation. Cows previously synchronized and not found pregnant were randomly assigned in two resynchronization protocols (Day 0); one with the classical Ovulation synchronization (Ovsynch) protocol without progesterone (P4) supplementation (n=66) and the other with exogenous P4 (PRID® alpha, Progesterone 1.55 g; CEVA Tiergesundheit GmbH) from Days 0 to 7 (n=67). Progesterone was measured on Days 0 and 7 of the resynchronization protocol as well as 4 and 5 days post insemination (p.i.). Progesterone supplementation raised the P4 levels on Day 7 (P<0.05), but had no (P>0.05) overall effect on resynchronization rates (RR) or pregnancy per AI (P/AI). However, cows with Body Condition Score (BCS) >3.5 showed increased P/AI and cows with BCS<2.75 decreased P/AI after P4 supplementation. Primiparous cows had higher P4 values on Day 7 than pluriparous cows (6.02 ± 1.49 ng/ml vs. 4.67 ± 0.54 ng/ml, P=0.04) and tended to have higher RR (90.5% vs. 76.9%, P=0.06). In cows having P4 values above 1.0 ng/ml on Day 0, higher RR were noticed in comparison to cows having lower P4 values (88.6% vs. 50% respectively, P=0.02). This study indicates that the supplementation of progesterone in resynchronization protocols has minimal effects on the outcome of these protocols. Parity had an effect on the levels of circulating progesterone at initiation of the protocol, which in term influenced the resynchronization rates.