Purpose Cancer drug prices at launch have increased in recent years. It is unclear how individual drug prices change over time after launch and what market determinants influence these changes. We measured the price trajectories of a cohort of cancer drugs after their launch into the US market and assessed the influence of market structure on price changes. Methods We studied the changes in mean monthly costs for a cohort of 24 patented, injectable anticancer drugs that were approved by the US Food and Drug Administration between 1996 and 2012. To account for discounts and rebates, we used the average sales prices published by the Centers for Medicare and Medicaid Services. Costs were adjusted to US general and health-related inflation rates. For each drug, we calculated the cumulative and annual drug cost changes. We then used a multivariable regression model to evaluate the association between market and cost changes over time. Results With a mean follow-up period of 8 years, the mean percent change in cost for all drugs was +25% (range, -14% to +96%). After adjusting for inflation, the mean cost change was +18% (range, -16% to +59%). Rituximab and trastuzumab followed a similar pattern in cost increases over time, and the inflation-adjusted monthly costs rose since approval by 49% and 44%, respectively. New supplemental US Food and Drug Administration approvals, new off-label indications, and new competitors did not influence the annual cost change rates. Conclusion Anticancer drug costs may change substantially after launch. Regardless of competition or supplemental indications, there is a steady increase in costs of patented anticancer agents over time. New regulations may be needed to prevent additional increases in drug costs after launch.