Although hospital consolidation within markets has been well documented, consolidation across markets has not, even though economic theory predicts—and evidence is emerging—that cross-market hospital systems raise prices by exerting market power across markets when negotiating with common customers (primarily insurers). This study analyzes hospital systems using the American Hospital Association Annual Survey Database and defines hospital geographic markets as commuting zones that link workers to places of employment. The share of community hospitals in the US that were part of hospital systems increased from 10 percent in 1970 to 67 percent in 2019, resulting in 3,436 hospitals within 368 systems in 2019. Of these systems, 216 (59 percent) owned hospitals in multiple commuting zones, in part because 55 percent of the 1,500 hospitals targeted for a merger or acquisition between 2010 and 2019 were located in a different commuting zone than the acquirer. Based on market-power differences among hospitals in systems, the number of systems in urban commuting zones that could potentially exert enhanced cross-market power increased from thirty-seven systems in 2009 to fifty-seven systems in 2019, an increase of 54 percent. The increase in cross-market hospital systems warrants concern and scrutiny because of the potential anticompetitive impact of hospital systems exerting market power across markets in negotiations with common customers.