Changes in competition in selected US domestic airline markets since deregulation of the airline industry are examined in this paper, focusing on both the top 100 markets and the largest markets out of dominated cities. Our analysis of carrier concentration in origin-destination markets since 1978 shows an increasing trend in concentration in recent years, after an initial period of decreases. The average concentration levels in the top 100 markets were still lower in 1991 than in 1979. At the same time, average concentration levels in 150 markets out of ‘dominated’ cities had returned to 1979 levels by 1991, after reaching a low point in 1985. Both the increases in competition that occurred during the first years of deregulation and the subsequent decreases can be explained by two structural changes experienced in the US airline industry: the numerous mergers of large carriers during the late 1980s and, more important, the development of extensive hub and spoke networks by virtually all airlines.
