There are quite a number of fungal and other pathogens that can cause leaf browning, so it's pretty hard to tell exactly what's going on. It depends on the kind of hydrangea, the various cultural practices (weeding? feeding? mulching? pruning? etc) as well as variables like temperature and  moisture. One needs to follow the progress of a problem usually to figure out specific issues, since many problems can have brown leaves at some point. Here are a couple of reputable sources that might help you narrow the problem down. It would be best to have some local who can look at your plants and their context to make a better diagnosis. http://www.usna.usda.gov/Gardens/faqs/hydrangeafaq2.html and http://www.aces.edu/pubs/docs/A/ANR-1212/ANR-1212.pdf