Booking air ticket without middle name

I booked on United using our Mileage Plus profiles and DH had not included his middle name -- he called United and they were able to add the middle name (no charge) and also put a note in the record for Lufthansa for the return trip. I don't know whether we got preferential treatment because we were flying on miles. It worked fine (about three years ago.)
I had heard that correct first name and correct last name is enough. Check with the airline ??

You will be fine. First and last names must match. My middle name is rarely on my ticket. I have never been questioned or stopped.

I can confirm what Frank has said, no requirement for middle name.

Some inner European airlines have an issue with wrong and incomplete names due to security guidelines, normally all airlines shall have.

Which airline is this?
Middle names are not universal in Europe, and in some countries rare (I don't have one), or never used. Websites for European airlines usually do not have a space to enter a middle name, I imagine it is the same for a travel agent booking.

My first name is longish (I go by my middle name) and I've noticed when I print my boarding passes, that my middle name usually just shows up as the first letter or two.

Which airline? Delta let me add middle names for my kids last year, no penalty.

Ryanair, Wow and Easyjet accept small changes of name data (e.g. typer) for free.

Instead of more complex changes it might be cheaper to buy a new ticket (source Ryanair name change policy).

I never use my middle name on my airline reservation and have never had a problem. I have flown on Norwegian, British Air, Air Canada, American Airlines, Smartwings, EasyJet, Czech Airlines, Virgin and probably a few others I can’t remember right now. First name and last only on the ticket. My passport does have my middle name, I just don’t use it and have never been questioned about it. Have a great trip!

However, it might be a problem if you have Global Entry and want to use it on your upcoming trip, and the name on your Global Entry registration does not exactly match the name on your airline ticket. For the first time in a number of years I recently did not have TSA-pre on my boarding pass. I was flying Southwest, and turns out my Southwest Rapid Rewards account - and thus my SWA ticket - only had my middle initial, and according to Global Entry the names must match exactly. (I have my full name, no initials, on my Global Entry account.) I called SWA to add my full middle name to my RR account. Hopefully all will be well next time I fly them. (Yes, I know that sometimes Global Entry folks can randomly be denied TSA-pre on their boarding pass. However, just in case it was a problem with the names not matching, I hope that that has been remedied.)

Thanks everyone for your replies. The flights were booked thru Delta Vacations but changed to Air France just last week. When I called Delta myself to ask to add the middle names to match our Passport, they said they could not do so after the "ticket #" was issued. Rebuying the full Delta Vacations package would be very expensive. Travel Agent says middle names are often not on tickets - 1000s per year from their Agency. Based on your replies, I guess I will just trust it!

My passport includes my full middle name. When booking my middle name is in the booking. It is good news there may be a way to add middle name if I did not originally add it to the booking. Over 15 years, my boarding pass may have an initial or not, never the 3 letter middle name. I can hardly wait for when I arrive at airport to be denied boarding. It has not happened, yet!