I hate being called Madam. I feel like I should be running a brothel. I'm not fond of honorifics in any case. You can be polite without using them. The way Americans call each other 'sir' as they go on to disrespect them is ridiculous.

Thank you all for your considered opinions. Last night I used 'Madam'. At first it sounded odd to me but I am sure it was ok for our female guests. It's great to see that on this site there are so many people who will respond so thoughtfully.

This depends on the context; culturally and geographically. Madam is the more Anglicized version of French Madame. Many restaurants have adopted French and pseudo French sounding terms to add to their also French adopted AMBIENCE... Like CUISINE, for example. How often do we see 'Italian cuisine', Mexican Cuisine', 'Thai Cuisine' or 'Hungarian Cuisine'.. get the idea? Why not Mexican cocina or Italian cucina? That's an easy answer. When you call it cuisine you can usually charge more.

It's all rather pretentious and I can understand why anyone NOT serving in a French restaurant might be uncomfortable using Madam. In American English a madam has an entirely different and degrading meaning anyway. Your question omits a description of the type of restaurant. Is it a chain or coffee shop level place or a 'finer dining establishment' or a real swell joint?

You have to 'size-up' your audience. What do THEY expect? Depending on age/s and social strata of clients, madam might be expected or considered a bit much. In the best restaurants your party will be assigned to you and the maitre d' will make an introduction as he/she turns you over or at least tell you the name (surname) of the party as he/she bolts back to the door. It helps in avoiding priggish ways of addressing people so you can modernize with Mr., Mrs., Miss etc. adding their surname in place of the more slavish, impersonal Your Highness, Your Majesty or Madam.

In USA one can often get away with warm smiles and dropping names etc. altogether. Just ask the question the same way you would a brother cousin or sister. Good Evening. Welcome. So good to see you. Care to sit here? So and So will be right along. We're so pleased to see you (again) or 'we've missed you'! Who needs the extra titles added anyway? A genuine welcoming smile will cover most bases.

This is all Frayed Bear's fault.

@Silver1wun should be able to advise you.

There again it used to be in Lancashire that the word "Luv" was perfectly acceptable with strange women of all ages though with children could be prefaced with "little".
In Australia male and female are frequently assaulted with the honorific "Mate" which can also be imbued with tones of sarcasm implying the one addressed is also stupid.

Madame and Mademoiselle can both be correctly shortened to Ma'am.
If you want to be old fashioned you can also use M'lady.

It's often shortened to "ma'am," actually, which is more commonly used. In the military, terms of respect for your superiors are "sir" and "ma'am," which have always served me well in public-facing jobs.

I would think that the term 'madam', would be appropriate; however, I've been witness to a few Irish women, here in the U.S., that strictly forbid to be called such, or even "ma'am", as it refers to them being heads of whore house establishments(?)
My grand son's caregiver made this QUITE clear, upon our first meeting with her, to set up day care for both my grand son's, at the time. She was adamant on this, and my daughter and I understood completely.

If there is more than one female I would start by saying something like, "Good evening ladies". If there is just one female I would address her as "Madam".

Of course @Marionville is correct but what a wonderful word "Madam" is for imbuing with sarcasm when you encounter an ignorant low bred guttersnipe and trollop. ?

Madam is correct.. it is polite and better than not addressing the ladies with anything...there really is no substitute.