Is this a sewer odor? or a mold odor? Was there mold on the tank lid?

I would not think that water on the tank lid would cause odor. I would be inclined to think that you have a leak in the waste pipe. But I am no plumber :)

Thanks Enduring. Its not a sewer odor. More of a musty/mildew smell. And there is no mold on the underside of the tank lid; it just gets wet. The toilet is brand new and is only used by me. There are no men in my household. Never had this issue with my previous toilet which was 16 years old. I will definitely call a plumber.

I know this is a old post, but were you able to get the problem resolved? Do you have an anti-sweat valve installed for this toilet?

I, too, would be interested in learning the resolution to the sweating tank lid issue. While I haven't had any unusual odors with any of my four Toto toilets, I have experienced water dripping from the underside of the tank lids any and every time I've removed them. So IF you were able to learn about the cause in your situation, I'd be grateful if you'd pass that information along. :-)

@Brendan: Can you please explain how an anti-sweat valve works? Thanks!

P.S. Here's a pic of one of MY lids, with condensation on the underside only (note the little puddles of water on the toilet seat lid, which is what the tank lid is sitting on in this photo):

Samantha, in my opinion, anti-sweat valves, also known as tempering valves are used mainly for older toilets that sweat on the exterior surfaces, especially around the uninsulated tank. In summertime, when older toilets flushed, they drain most of the water from the tank, and upon being refilled, the incoming water is colder than the warm humid air in homes that do not have air conditioning so you will see condensation of the surface of the tank, and sometimes the bowl. As long you have air conditioning or a modern low flush toilet, you do not need an anti-sweat valve. Those type of valves mix a bit of hot water with the cold water to the toilet so it is more to room temperature to eliminate the "sweating".
In both of our case, we have the newer toilets which are designed not to sweat by only flushing roughly half of the water in the tank, so that the incoming cold water will mix with the temperated water.
The problem we seems to have is that the tank water was "too" warm and it steamed or evaporated causing condensation underneath the tank lid. Originally, I had assumed the anti-sweat valve was at fault, that the adjustment screw was set too high for the amount of hot water to mix with the cold water. But I found out recently, there was no tempering valves installed in my case, it turns out my client has a hot water recirculating system where the cooled hot water is returned to the heating tank by the cold water lines in her house. Somehow some hot water got into the tank. Do you have a similar system in your case? Check the temperature of your tank water, is it warm?

I'm unfamiliar with the recirculating system you describe your client as having. I need to look that up!

As for my system, it's the standard: cold water only. I do have a theory, though, as to what could be causing the dripping tank lids in my house...and it has nothing to do with condensation. I think it may be the G-Max flushing system, which blasts the contents of the bowl down the drain. And it's that blast that is causing the water in the tank to splash up, hitting the underside of the lid. Now it's possible that I'm totally wrong, but, it gives me something to Google. :-)

I have a Toto Lloyd. Initial one leaked due to hairline crack, and needed to be replaced. No problems since. It's been about 2 years; I love it.

My TOTO toilet was install about 10 years ago. It has worked great until recently when the tank and the lower part of the toilet bowl started to sweat to the point that it's ruined my wood floor. What can I do to fix the problem?