Blossom end rot. Sometimes it doesn't show up on the exterior.

The FAQ has a good write-up.

I recieved some Plum tomatoe plants from a friend. He says he got the seeds from an older fellow that said the seeds come from Italy. The Plums are not the same exact shaoe as those you buy from a greenhouse or grown from seed. The fruit is rather long, the biggest I have is almost 4 1/2 inches long with a point at the bottom rather than rounded. The problem I'm having is a lot of the fruit has brown spots on the bottom which as it rippens rots. He has the same problem though. He waters his frequently, I on the other hand once every three days. He says he talked to his friend and was told that it was a calcium deficientcy in the soil. I sort of thought it might have been something missing in the soil, but does calcium sound right to anyone. I'm just a part timer for the fun of it, any help would be appreciated. Thanks in advance.

MY TOMATOES ARE ROTTING FROM THE BOTTOM. I DON'T KNOW WHAT THE PROBLEM IS. IT HAS NEVER HAPPENED BEFORE.

Blossom End Rot. It happens. See the FAQ from the Tomato forum:
http://faq.gardenweb.com/faq/lists/tomato/2000082444023571.html

By the way, the sprays sold for Blossom End Rot are a waste of money.