Doctors found "two spots" on Michael Lohan's lung after he was rushed to hospital with a suspected heart attack.
Lindsay Lohan's father was spending time with his sons, Landon, two, and 10-month-old Logan, at the Florida Department of Children and Families office when he was taken ill and was shocked by what the doctors found.
He told Mirror.co.uk: "I was on my visit with the boys at DCF. I got chest pain and started sweating profusely. My mouth got very dry and the pain in my chest got worse.
"I tried to stand up but I couldn't. By nitroglycerin was at my house so I called my mother to bring it to me and just when she got there and I took one the paramedics arrived.
"The chest x-rays and tests show that I have a constriction to my arteries but they also found two spots on my lung. This might have been a blessing in disguise."
Michael and his sons' mother Kate Major have a famously volatile relationship and last month his mother was appointed a temporary guardian of the boys after the pair had a massive bust-up.
Michael added: "I've been so stressed over the situation with losing my kids and what Kate has done that is taking a toll on me.
"But I won't let it stop me from fighting for my kids back and doing the right thing. Is faith that God will get us through this."
Michael's collapse came just before he was due to appear in court to discuss the custody arrangement.