Been in a small pot a long time and these become big plants so is it pot bound and or short of water? My guess is it needs to be planted in the garden and given space to grow.

In my experience the larger (older) fatsia leaves do go yellow and then brown before falling off,its just older leaves making way for the new growth which will be appearing now.The plant in the photo looks ok otherwise and i found they do well in pots for a while but do grow quicker when in the ground with some shade.

Not sure the pot's big enough for a plant that size, might be worth turning it out to see if its completely potbound. If it is, put it in a larger pot.
If the leaves have only just started wilting in the last 4 days, and your plant gets sun between 11 and 3, it could just be the heat of the sun - its a bit of a shock to most plants, following on from such a cold, dank and dull April and May.

I realy know it could do with being turned out,but with experience I have learnt that shaped pots, as this is,(lovely as they are) are a real pain to empty out so have been puting it off.I have had yellow/brown leaves before but nothing like this,its the new leaves that seem to be wilting.It IS in sun all afternoon,and will "ask "for water but never seen this before.Hopeing thats all it is Bamboo a shock to the system cos I too am wilting!!!(:0)

I've noticed some plants in the garden are wilting by mid afternoon, really badly -yet they're mature shrubs, and will have deep roots right down where it must still be wet from all the rain, so I'm concluding its the shock of sudden very hot sun, the leaves haven't had time to adjust. What a year, first they come out and get caught by frost and cold, they acclimatise to that and now they're being caught by sunburn and dehydration...