An answer to a related question posted a four-panel comic:
Breaking Bad: Anywhere But America Edition
Doctor: Mister White... I’m afraid you have incurable lung cancer.
Walter White: But... How will I pay for my medical treatments? This will bankrupt my family!
Doctor: Nonsense! The government will pay your medical bills, taxpayer! What kind of barbaric society would allow medical care to hinge on a person’s wealth?
Walter White: Oh, phew. In that case I guess I’ll go back to teaching chemistry.
The End
This got me thinking: Walt was employed by the local public school system and had an insurance plan. Is it ever directly stated in the show that he was denied coverage? Looking at the Season 1 transcripts there's only two mentions of "insurance". Once in Episode 1:
Walter: This is so embarrassing. I am fine. Honestly. It's just some bug going around. First my wife had it, then my son, and now me. It's just like a chest cold. Could be some low blood sugar as well. I didn't have the greatest breakfast this morning, honestly. Hey, listen, can you do me a favor? Can you just drop me off at a corner somewhere?
EMT: No. Sorry.
Walter: It's just that I don't have the greatest insurance
And then once in Episode 5:
Skyler: The lady from the billing office said they usually bill the insurance company directly, but since our HMO is not paying for it, we can pay up front with a credit card.
Walter: Good.
Skyler: And then Elliott will send a check.
Walter: I will take care of it.
Was it that the insurance refused to pay for treatment outright? Or was this because Walt wanted to get treatment at a more expensive clinic that wasn't covered by his plan?