I.B.M. has been quietly saying for months that it planned to enter the market for electronic health records, a business being primed with hefty government subsidies. The only question has been how Big Blue, a company geared to deal with big corporations, would cater to the messy, fragmented market of physicians. I.B.M. provided its answer Thursday morning. It’s not going in alone, but as a technology partner. The offering will be a Web-based subscription service, but enhanced by I.B.M. data-mining technology and tweaked by the scientists in its research labs. And I.B.M. is going into electronic health records selectively, and not yet marketing to small practices with just a few doctors. I.B.M.’s partner is a subsidiary of the insurer Aetna, ActiveHealth Management, which makes disease management software and personal health records. ActiveHealth will provide the user record and decision support technology, while I.B.M. provides the cloud computing services and data-intelligence technology.