Chicago's public pension fund was rated the worst of 50 plans recently evaluated by Moody's.

The ratings service ranked the funds based on their adjusted net pension liabilities when compared to their net operating revenue. Chicago, based on 2011 data, had a ratio of 678 percent, Moody's said. The second-worst fund was Cook County, the home of Chicago, which came in at 382 percent.

The liabilities faced by public pension funds have gotten more attention recently as agencies like Moody's and Standard & Poor's have begun to include them in calculating bond ratings. In July, Moody's lowered Chicago's rating from A3 to Aa3. This month, S&P changed the city's outlook to negative, mostly because of the increasing amount of revenue being diverted to fund future pension liabilities.