Northwick Park Hospital, in Watford Road, has reopened its Accident and Emergency department this afternoon.
However GP referrals are being transferred to Central Middlesex Hospital, in Park Royal.
Children are not being admitted, but the hospital is hoping to reopen to children by the end of today.
Chief executive of The North West London Hospitals NHS Trust, Fiona Wise, said: "Our main priority is to try and ensure as little disruption for our patients as possible and that those who were moved yesterday are comfortable.
"Generators are up and running and some parts of the hospital will continue to be supplied by these for some time.
"We will be making a full assessment of areas and are working towards getting as fully operational as possible in the next 24 hours and I will update staff and patients later today regarding exact timings."
The outpatient department, maternity unit and mental health unit are open. Patients with appointments at St MarkÕs and cardiology are advised to go to the outpatient reception.
For more information call 020 8869 2444 or visit website www.nwlh.nhs.uk.