A solicitor representing victims of a drugs trial which went wrong claims the men could now develop cancer and other serious illnesses.
The drug trials took place at Northwick Park Hospital in London
Six men were left seriously ill after volunteering to take part in the trials at Northwick Park Hospital in north-west London earlier this year.
Martyn Day, a solicitor for several of the men said they now had to face "the most terrible future".
Parexel, which arranged the trials, says all guidelines were followed.
It is a clinical research organisation which carried out the trial on behalf of TeGenero, a German pharmaceutical company.
One of the trialists, Nav Modi, found out that his immune system was likely to be unable to fight off serious illnesses after Mr Day arranged for independent tests to be carried out on him and three other of the men.
"I don't know what's going to happen the next day," he said.
"Every time I go to bed I feel that I won't be able to see the sun tomorrow."
Mr Modi suffered multiple organ failure following the trials of the drug TGN1412 and is now seeking compensation.
Mr Day, who is acting for Mr Modi and some of the other trialists, said: "The reality is that they're going to have to face the most terrible future prospect of the most serious cancers happening at any time, the prospect of serious diseases, the most terrible, terrible time."
The Medicines and Healthcare products Regulatory Authority (MHRA) has previously said Parexel failed to follow proper procedures.
It found there was no contract in place between TeGenero and Parexel at the beginning of the trial.
TeGenero has maintained that the men's reactions were "completely unexpected" and did not reflect the results obtained from the earlier laboratory studies.