Leaves turning brown and stems dying may be watering issues, specially since you planted it this year and summer has just started. Check the basics:

1. Water - water frequently in the mornings so as to keep the soil moist and be aware that you may have to do this almost daily during the plant's first summer. The soil should feel moist if you stick a finger to a depth of 2 inches. If it feels wet, cut back on watering. If it feels dry then water. Make sure that you are also watering the root ball since the plant has not had time to grow big roots yet. At the same time, try not to water the leaves too much (aim for soil). Because this plant will eventually have a large root system, you need to mulch it well: about 2-3" of mulch about a foot past the dripline.
2. Sun - it should be getting a few hours of morning sun
3. Soil - the soil should drain well
4. Fertilizer - in northern climates, some manure in June or some cottonseed meal is all you need. In southern climates, we feed it in May and July.
5. Planting - many hydrangeas are pot bound (root bound) in the pots that they are sold.

A little damage to the leaves from insects is normal and should not kill the plant. But to take any action, you have to stop by at night and identify the culprit (or capture one in a sealed transparent plastic bag so someone at a local nursery can identify it for you).

Luis

It's been hot as heck here, with temps wiwthin a reasonabalae ranage for only the past few days. So, I suspect your hydrangea has run short of water in the original rootball.

Beyond that, temporary shade will slow water loss during the heat -- which they say may be returning shortly.

Thanks for the advice. I have been only watering it every 2 or 3 days--I will do it everyday. I realized in my message, I wasn't clear about the leaves--in addition to turning brown, there are chunks missing out of individual leaves and the brown dying stems have been completely stripped of leaves. Could this be a slug or insect? having just moved here I have not had experience with slugs.

Mine's doing the same thing...it was only planted in April, but the automatic sprinkler just misses the plant itself. Now the leaves are turning brownish and curling....I'm hand-watering it...will it be okay do you think?

Ggrumke, hydrangeas have very few pests that eat its leaves. Not even japanese beetles seem interested. How much leaf damage are we talking about here? I usually do not bother with some damage. Or if I discover a few leaves partly eaten but no additional damage afterwards, I just let things be as they are. Are you continuing to get more leaf damage?

Mizliz, hand watering is better than the sprinkler because you can water the soil deeply better than any sprinkler. Besides, the sprinkler actually waters the leaves mostly and this can cause fungal infections in the leaves. Water only when the soil feels almost dry to a depth of 3-4". Mulch it heavily, four inches say, past the drip line as its root system will be quite big compared to other hydrangeas. Just do not forget to water it since there is no "back up system" for times when you are out of town. Can you extend the sprinkler system or install drip irrigation to cover that area?

Luis