ABSTRACT This paper investigates the role of bank capital regulation in risk control. It is known that banks choose portfolios of higher risk because of inefficiently priced deposit insurance. Bank capital regulation is a way to redress this bias toward risk. Utilizing the mean‐variance model, the following results are shown: (a) the use of simple capital ratios in regulation is an ineffective means to bound the insolvency risk of banks; (b) as a solution to problems of the capital ratio regulation, the “theoretically correct” risk weights under the risk‐based capital plan are explicitly derived; and (c) the “theoretically correct” risk weights are restrictions on asset composition, which alters the optimal portfolio choice of banking firms.