The American Diabetes Association, by contrast, reported receiving more than $28 million in industry funding in 2014, or about 15 percent of its budget, but provided detailed disclosures of which companies donated, and how much, the study authors said.

In a statement, the hemophilia foundation said it never allows its corporate sponsors to influence its decision-making, and that it also does not endorse specific products or favor certain companies. It declined to provide precise dollar amounts of contributions from companies, saying that the foundation complied with “accepted financial reporting standards.”

The study’s authors said transparency could be improved by requiring the drug and device industries to report how much they donate to patient groups, much like they are already required to do with doctors.

That was applauded by other critics of the drug industry. “I think sunshine is an excellent disinfectant,” said David Mitchell, the founder of a new group, Patients for Affordable Drugs, that seeks to lower drug prices, and does not take funding from industry groups. He was not involved in the study.

Mr. Mitchell said patient groups often do not disclose that they take industry funds when they testify before Congress or government agencies, or when they disseminate educational information to patients.

Many have also been silent on the issue of rising drug prices, even as the issue has enraged patients, who have been increasingly exposed to the prices that pharmaceutical companies set as insurers have asked them to pay a greater share of their drug costs. Last summer, patients and their families loudly protested the skyrocketing price of EpiPens, though the movement gathered steam on social media rather than through traditional patient-advocacy groups.

And a year ago, for example, a representative for the National Psoriasis Foundation did not disclose that her group receives at least 40 percent of its annual revenues from drug companies when she testified before the North Carolina state legislature on an unsuccessful measure supported by the pharmaceutical industry that would have limited insurers’ ability to block coverage of certain drugs. Similarly, the hemophilia foundation did not disclose its pharmaceutical ties when it took the industry’s side in 2015 in a letter to the Food and Drug Administration over the issue of biosimilars, which are cheaper alternatives to complex biological drugs.