The survey, conducted after enrollment in Obamacare plans closed in April and released on Thursday, is the first comprehensive study addressing the health of those who have purchased this new insurance, created by the Affordable Care Act.
About 8 million people purchased Obamacare plans, more than half of them qualifying for government subsidies toward their monthly premiums based on household income.
The relative health of policyholders is a key factor for insurers when they set premium rates, which are designed to cover the costs of medical services among participants. Kaiser said it was unclear how this finding might influence rates because insurers had been anticipating a sicker-than-average customer base.
The telephone survey of a nationally representative sample of 742 people also found that 57 percent of the new Obamacare plan members had been uninsured just before they obtained coverage. Most of them said they had not had coverage for at least two years.
Insurers including WellPoint Inc and Aetna Inc have said they are still gathering data about their new customers, many of whose coverage did not start until May 1. Insurers have already submitted many of their 2015 rates to regulators.
The survey found that 16 percent of respondents with Obamacare plans purchased on state exchanges rated their health as "fair," the fourth-worst of five levels offered. That compared with an average of 11 percent for all people who bought individual insurance.
About 4 percent of people buying from the Obamacare exchanges rated their health as "poor," compared with 2 percent in individual plans sold outside of those exchange and less than 0.5 percent of people who still hold plans that do not comply with the law. These plans are expected to phase out of the market in the next three years.
At the opposite end, 21 percent of people in Obamacare exchange plans said they had "excellent" health, compared with 24 percent of all individual plan holders.
Twenty-seven percent of Obamacare plan members said they had "very good" health, compared with 31 percent overall, and 31 percent said they had "good" health, compared with 30 percent overall.
Kaiser found the 55-to-64 age group was the largest among members of ACA-compliant plans sold both on and off the exchanges. People older than 65 are eligible for the government's Medicare program.
Of the other holders of ACA-compliant plans, 17 percent were aged 18 to 25, and another 17 percent were 26 to 34. About 19 percent were 35 to 44, and 21 percent were in the 45-to-54 age bracket.
The survey was conducted April 3 to May 11.