As we noted in our earnings release, in October 2010, the Financial Accounting Standards Board issued new guidance concerning the accounting per cost associated with acquiring or renewing insurance contracts. We have adopted this guidance effective January 1, 2011, and have therefore adjusted our previously issued financial information. Adoption of this guidance reduced the carrying value of our deferred acquisition costs as of December 31, 2010, by $78.7 million, and Tower Group Inc.'s stockholders equity by $42.6 million. Diluted earnings per share for the first quarter 2010 were reduced by $0.09 as a result of this change in accounting. A replay of this call will be on the Tower website immediately following the call. With that, I'd like to turn the call over to Michael.
Thank you, Bill, and good morning, everyone. I'd like to thank all of you for joining us on this conference call to discuss our first quarter operating results. As Bill mentioned at the beginning of this call, we will refer to the presentation that is available on our website during this call. In addition to providing our quarterly results, we provided additional information and color in the presentation material that we believe might be helpful in demonstrating the positive trends that we're seeing in our core business. On this morning's call, I will provide you with updates in several areas of our business, including our views on the insurance market, our strategic response and new business initiatives that will enable us to generate organic growth. Bill will then provide a detailed overview of our financial performance. We will then conclude this call with a question-and-answer session.
As described in last night's press release, Tower Group, despite losses from an increase in winter weather-related claims activity, produced another quarter of profitable underwriting results. As shown on Page 2, our operating income increased by 52% to $20.3 million in the first quarter from $13.4 million during the same period last year. Our diluted operating EPS increased by 69% to $0.49 per share compared to $0.29 per share during the same period last year. As we previously announced, we experienced an increase in winter weather-related claims activity from the record snowfall in the Northeast, Midwest and Mid-Atlantic, beginning in late December 2010 and throughout the first quarter that totaled $9.8 million on an after-tax basis or $0.23 per share. Excluding the storm losses, our first quarter 2011 net income and EPS would've been $30.1 million or $0.72, respectively. As Bill mentioned at the start of the call, we adopted the new accounting rules applicable to the treatment of deferred acquisition costs beginning on January 1 of this year, which required us to adjust our last year's financials using this new accounting standard. As a result, we're seeing greater percentage increases and positive trends in 2011 as compared to 2010. Bill will provide further details on the effect of this accounting rule change later on in this presentation.