Everything is relative. I'm very sensitive to cold. I'm planning on wearing long pants and short-sleeved shirts and having a sweater just in case. This site might be helpful:

www.worldclimate.com

and what do you wear in September where you live? What do you call summer clothes? or fall clothes? for Nome? or for Bakersfield?

and where in Italy are you going?

In Sept 99 I spent 3 weeks in Florence, Tuscany and Venice. It was in the 80s and sunny almost everyday. The only time I needed lont sleeves/jacket was the 3 nights I spent on Lake Como at the end of my trip-cool and rainy. Have a great trip.
Jean

Victoria,
The previous posters are on the right track. Lightweight layers. The days will be warm, the nights may be a bit cooler, so plan accordingly with a lightweight jacket or sweater.
Buon Viaggio,
BC

Even as late as November, if you're in Rome or south, the days can be very warm short sleeve-ish weather, with cooler evenings. I was too warm, but I'm from New York/Chicago, and am ready for snow by November!

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Thanks for your help everyone. I should of mentioned I live in Florida and have a lot of light colored summer like things and since we will be in Rome, Sorrento and Florence I did't know if light colors would be best or darker colors. I just don't want to pack the wrong things. Thanks again..

From everything I've read on this forum, go dark! (this is not the voice of an experienced Italy traveller, just a reader of the board). Will be going to Italy in late September and am bringing primarily darker clothing.

Thanks everyone..

DJ , That's what I really needed to know. Light or Dark. Do they go by the after labor day rule in Italy too...

Diane
Labor Day in Europe is usually May 1, so the beginning of Sept doesn't have a clothing turning point.
When I've been in Italy in late Sept early Oct I've done what others suggested and brought light layers.
Mornings in both Florence and Venice
I found to be cool (upper 50s to low 60sF) but by
noontime I couldn't wait to get rid of my sweater or jacket. Days with drizzle will of course feel cooler.
Dark cottons and dark linens will work fine. and bring an umbrella.

Comparing the color scheme in Italy vs. Florida --- they wear more dark colors year round than what you'll see in Florida. During the month of September, I'd suggest more summer weight fabrics. However I'm not a native southerner, so my perspective may be different than yours.