Specialty drugs are used to treat complex or life-threatening conditions, often at high financial costs to both patients and health plans. Three states - Delaware, Louisiana, and Maryland - passed legislation to cap out-of-pocket payments for specialty drugs at $150 per prescription. A concern is that these caps could shift costs to health plans, increasing insurance premiums. Estimates of the effect of the caps on patient and health-plan spending could inform future policies.We analyzed a sample that included 27,161 persons under 65 years of age who had rheumatoid arthritis, multiple sclerosis, hepatitis C, psoriasis, psoriatic arthritis, Crohn's disease, or ulcerative colitis and who were in commercial health plans from 2011 through 2016 that were administered by three large nationwide insurers. The primary outcome was the change in out-of-pocket spending among specialty-drug users who were in the 95th percentile for spending on specialty drugs. Other outcomes were changes in mean out-of-pocket and health-plan spending for specialty drugs, nonspecialty drugs, and nondrug health care and utilization of specialty drugs. We compared outcomes in the three states that enacted caps with neighboring control states that did not, 3 years before and up to 3 years after enactment of the spending cap.Caps were associated with an adjusted change in out-of-pocket costs of -$351 (95% confidence interval, -554 to -148) per specialty-drug user per month, representing a 32% reduction in spending, among users in the 95th percentile of spending on specialty drugs. This finding was supported by multiple sensitivity analyses. Caps were not associated with changes in other outcomes.Caps for spending on specialty drugs were associated with substantial reductions in spending on specialty drugs among patients with the highest out-of-pocket costs, without detectable increases in health-plan spending, a proxy for future insurance premiums. (Funded by the Robert Wood Johnson Foundation Health Data for Action Program.).