This analysis finds that marketplace health insurance premium levels and premium growth directly correlate with the number of insurers participating in a given rating region. Median monthly benchmark premiums range from $270 in markets with six or more insurers to $451 in markets with just one insurer in 2017. Likewise, in regions with 6 or more insurers, the median 2017 benchmark premium increase was 5.0% compared to 29.8% in regions with only 1 insurer. We also find that markets with only one or two insurers tend to be heavily concentrated in southern states and sparsely populated areas. The median population size for rating regions with only one marketplace insurer is 148,000, compared to 1.1 million in rating regions with 6 or more insurers. Nearly equivalent shares of the U.S. population live in areas with five or more insurers as one or two insurers.