For the fourth consecutive year, the state’s largest insurer of medical liability for doctors said it will not increase the base premium rates on policies.

ISMIE Mutual Insurance Co. credited its decision to a stable “lawsuit environment nurtured by Illinois’ 2005 medical litigation reform law, but cautioned that resulting market improvements could be at serious future jeopardy.”

Earlier this year, the Illinois Supreme Court struck down the liability law, again triggering a debate over whether caps on damages in the law were the reason premiums have been held in check. Trial lawyers and some consumer groups say premiums are volatile for a variety of reasons such as the stock market, which can affect reserve levels insurers use to pay claims.

The extension of ISMIE Mutual’s base rates affects annual policies renewing “on or after” Oct. 1 for the 2010-2011 policy year, the insurer said. It insurers 12,000 doctors in Illinois.

“Since enactment of the reform law, claims reported to ISMIE have steadily decreased, from 3,295 in 2005 to 2,379 last year,” said ISMIE Mutual chairman Dr. Harold Jensen.

“We believe Illinois’ reform law played a significant role in the decline in lawsuit filings,” Jensen added. “However, we are still very worried about high lawsuit awards which, in the long run, are not sustainable.”