The order clears the way for Juno to begin selling ad space on the service pending resolution of a lawsuit filed in December by rival NetZero, in which the ISP stands accused of patent infringement for displaying advertising pop-up windows.
The case is one of a pair of lawsuits the competitors have filed over advertising technology. In June, Juno filed a lawsuit alleging that NetZero and Qualcomm infringed on a patent that enables advertisements and other content to be displayed when a consumer is offline.
Analysts said the cases highlight the growing use of patent law to challenge competitors' business strategies.
"I think one of the questions of patents and the Internet age is can you patent not just the technology, but a business model?" said Lydia Leong, analyst at Gartner. "This is the usual legal spin that occurs when companies with similar business models start fighting patent wars."
Like NetZero, Juno offers a paid and an unpaid ad-supported Internet service. It said advertising represented a minority of its revenue last year.
On Thursday, Juno said that U.S. District Judge Stephen Wilson rejected NetZero's request for a preliminary injunction that would have broadened the existing temporary restraining order and extended its duration through the date of trial.
"We believe this was the right decision," said Juno Chief Executive Charles Ardai. "We don't believe that we were, or are, infringing any valid patent that NetZero holds. We thought it was inappropriate for our activities to be restrained in any way related to their claim, which we believe was untrue."
NetZero declined to comment.