Hi Jen,

You don't give any info on the leaves or crown of the tree, but I'm thinking either sycamore borer or canker stain.

Sycamore borer - see their page 10 and figure 26 here.
http://www.suddenoakdeath.org/pdf/PestAlert6.pdf

Canker stain
http://www.forestpests.org/sycamore/cankers.html

You may need a certified arborist that does diagnostics take a look at your trees or post your question in the Trees forum. Lots of folks there that know their trees.

Newt

The leaves appear normal, though they gradually thin from bottom to top. I would post more pix, but the county cut down my neighbor's tree that was 80% dead.

Jen