The United States Supreme Court issued an opinion saying that it was unlawful for police to take a blood sample of a man who was arrested on suspicion of driving under the influence of alcohol. The man in this case was pulled over by police officers who observed behavior that indicated that he was intoxicated before asking him to perform tasks for a field sobriety test.
The man did not perform the tasks to the satisfaction of police, so they attempted to obtain a breath test sample, which the man refused. After transporting him to a nearby hospital to get a blood sample that the man also refused to consent to, police decided to take the sample anyways. Taking a blood sample is considered a "search" for the purposes, and under the Fourth Amendment suspects have a right to be free from an unreasonable search and seizure.
In order to lawfully conduct a search without a warrant, the situation must qualify under one of several finite exceptions. In this case, the state court found that the situation did not fit within an exception to the warrant requirement, the more relevant of which would have been exigent circumstances requiring quick action by police in order to preserve evidence.
While it is clearly true that over time, the blood alcohol content in a suspect's blood stream will naturally dissipate, that is not sufficient to justify taking a blood sample without consent or a warrant.
A previous Supreme Court case asking a similar question found that a warrant was not needed to take a blood sample when there was an accident in which the driver and a passenger were injured.
Source: New York Times, "Court Rules Warrants Are Needed to Draw Blood in Drunken-Driving Cases," Adam Liptak, April 17, 2013.
Information about the rights of people who are suspected of driving while intoxicated is available on our Chicago criminal defense site.