There is unexplained variation in length of stay (LOS) across NICUs, suggesting that there may be practices that can optimize LOS. Three groups of NICUs in the California Perinatal Quality Care Collaborative were followed: (1) collaborative centers participating in an 18-month collaborative quality improvement project to optimize LOS for preterm infants; (2) individual centers aiming to optimize LOS; and (3) nonparticipants. Our aim in the collaborative project was to decrease postmenstrual age (PMA) at discharge for infants born between 27 + 0 and <32 weeks' gestational age by 3 days. A secondary outcome was "early discharge," the proportion of infants discharged from the hospital before 36 + 5 weeks' PMA. The balancing measure of readmissions within 72 hours was tracked for the collaborative group. From 2013 to 2015, 8917 infants were cared for in 20 collaborative NICUs, 19 individual project NICUs, and 71 nonparticipants. In the collaborative group, the PMA at discharge decreased from 37.8 to 37.5 weeks (P = .02), and early discharge increased from 31.6% to 41.9% (P = .006). The individual project group had no significant change. Nonparticipants had a decrease in PMA from 37.5 to 37.3 weeks (P = .01) but no significant change in early discharge (39.8% to 43.6%; P = .24). There was no significant change in readmissions over time in the collaborative group. A structured collaborative project that was focused on optimizing LOS led to a 3-day decrease in LOS and was more effective than individualized quality improvement efforts.