In addition, you may recall that in the first quarter 2011, our Libya project was still active and a positive contributor. Further, we benefited from the R&D tax credit extension in the US during that quarter. The balance of the difference was driven by non-operating items and 7% underlying growth in the business. Our first quarter gross revenue increased 5% over last year to $2 billion and net service revenue increased just over 1% year-over-year.
On an organic basis, gross revenue increased 4% driven by increased construction management activity while organic net service revenue was up 0.4% year-over-year.
Our total organic net service revenue grew 1.4% on a constant currency basis excluding the impact of Libya. We continue to see strength internationally in geographies such as Asia, Australia, Canada and Latin America and in our private sector business of commercial construction, environmental management and energy.
On the other hand, Western Europe remains a challenge. However we anticipate that our restructuring efforts will reap benefits in the second half of the year and we are seeing increased opportunities in this geography including recent sizable wins that are not yet reflected in the backlog. We also won 2.2 billion in new work during the quarter for a book-to-burn ratio of 1.1 and our backlog increased 2% from the prior year to $15.8 billion.Please turn to slide five. We would also like to highlight the cash flow performance in the quarter. Our cash flow from operations improved by $173 million year-over-year. This represents our best first quarter cash flow in five years and builds upon our record cash flow achieved in the fourth quarter. Our PTS DSOs came down by three days year-over-year. In total, our cash and cash equivalents increased by 20% year-over-year to $507 million and our leverage ratio declined to 1.4 versus 1.7 in the prior year. We continue to focus on cash flow as a key measure of operational performance and we’re on track to achieve our full-year target for free cash flow as a percentage of net income. Read the rest of this transcript for free on seekingalpha.com