Mike's illness obviously played a significant role in GFIII. But are there any indications in either of the first two films that he was diabetic? Or are we to surmise that this came on him in middle age? It's been decades since I read the novel, so is his diabetes mentioned there? I notice that Michael rarely drinks alcohol (except for wine). Should that have been taken as a clue?

I don't believe that we are given any indication of Michael's diabetic condition until GFIII.

I do remember Michael drinking alcohol in GFII.

Welcome, bradd!
I believe this question has been asked before.
The novel had no reference to Michael and diabetes. The only reference to any illness was a notation that Michael had been discharged from the Marines in WWII after suffering "a debilitating wound." In GFII, he swallows a pill during his train ride from Tahoe to Miami to see Roth. We don't know what the pill is for. A guess: diabetes was treated with insulin injections in those days. I'm guessing it was a Miltown, the popular tranquilizer of the era. (Senator Geary popped one, too, in Michael's office.) But, if memory serves, others here seemed to find hints of some illness earlier on.

Could Michael have been taking a pill because of his sinus condition created by the infamous McCluskey punch? I seem to recall in the novel that even after his surgery he still had sinus problems.

In Puzo's and FFC's minds, Michael was clearly diabetic by the end of Part 2. Here's a link to an early draft of their screenplay. If you scroll all the way down to the bottom, just before the flashback scene (originally scripted for Brando, not James Caan) there is a clear mention of Connie giving Michael an insulin shot in front of an 18 year old Anthony (obviously, another scene that was never made).

The Godfather Part II Screenplay - Early Draft

Pizzaboy, thanks very much for that link! It also adds another reason why Michael looks so bedraggled and played out in the final scene.

Welcome Bradd

I love to read the scripts, especially part 2, every so often. They really show the motivation of some of the characters and the points of view of Coppola and Puzo, that you may have otherwise missed.

Pizzaboy