The company said so far there was no evidence of theft or misuse of data.
LabCorp carries out millions of blood, urine and other diagnostic tests each week and is one of the world’s largest domestic commercial lab-testing companies, according to The Wall Street Journal.
Its database contains health information on roughly half the US population.
The WSJ said the ransomware that had hit LabCorp, which is based in Burlington, North Carolina, was a strain of the SamSam malware.
In a securities filing, LabCorp said its drug-development arm Covance had not been affected. But the WSJ said Covance had been affected as well, apart from tens of thousands of workstations, servers and devices.
It quoted a spokeswoman as saying Covance was affected by the deployment of security enhancements in other parts of the network.
In a statement, the company said it had "recently detected activity on its information technology network that was subsequently determined to be ransomware, which is an attempt to block or disable computer systems".
"LabCorp promptly took certain systems offline as part of its comprehensive response to contain and remove the ransomware from LabCorp’s network.
"This has temporarily affected some test processing and access to test results. At this time, there is no evidence of theft or misuse of data."
The breach comes in the same week that fears over breaches at Australia's My Health Record, an online health details database, resulted in a large number of people opting out.
LabCorp said: "We have been bringing those systems back up over the last several days. If you are experiencing delays in accessing your test results or getting a response to your email inquiries or phone calls, we appreciate your patience and apologise for the inconvenience."
Last year, hospitals in the UK were hit by ransomware known as WannaCry that badly affected Britain's National Health Service.