Since the Consumer Financial Protection Bureau opened its credit card complaint portal in Sept. 2010, more than 25,000 complaints have been filed with the CFPB. And while the 10 largest credit card issuers account for 93% of all those complaints, one company is responsible for more than 1-in-5 of all complaints filed with the Bureau: Capital One.

That’s according to the Ohio Public Interest Research Group’s new report [PDF] that analyzes some of the available data about the CFPB complaint portal.

With 5,625 complaints filed between Sept. 2010 and Nov. 2013, Capital One cards accounted for 21% of all consumer gripes. Citibank’s credit cards were the second most complained-about (4,514 complaints, 18% of the total), followed by Bank of America (3,320; 13%).

Problems with Capital One cards appear to be a nationwide issue, with Cap One receiving the most complaints from consumers in 43 states. In six of the remaining states — Connecticut, Maine, Maryland, New Mexico, New York, Utah — Citi earned the most complaints, with Bank of America being the most-hated card issuer in Alaska.

The complaint portal isn’t just for consumers to scream into a black hole about their credit cards. The idea is that the card company is supposed to respond to each complaint within a given time frame. Of course, the cardholders aren’t always pleased with the card companies’ responses.

If the consumer is unhappy with the card issuer’s response to the complaint, he can file a dispute. Once again, Capital One cardholders filed the most disputes (1,044), meaning about one out of every five Cap One complaints were disputed. This may have been the largest number of disputes, but it’s not the highest rate. That belongs to American Express, where cardholders disputed 26% of the resolutions suggested by the card issuer.

In terms of what people are complaining about, billing issues represented the largest percentage of complaints (18%), followed by gripes about interest rates (10%), identity theft (7%), credit reporting (7%), or closing and canceling accounts (6%).

In the end, the majority (60%) of all credit card complaints are resolved with merely an explanation from the card company. Complaints resulted in monetary relief 29% of the time, and with some sort of non-monetary relief in another 10% of cases.

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