An elderly couple and their son were threatened when raiders armed with weapons broke into their home.
Two men used the kitchen door to get inside the house on Coppull Moor Lane, Coppull, at around 8.20pm on Tuesday.
Inside the house were a man and a woman in their 90s and their 63-year-old son.
One of the burglars threatened the family with a crowbar and a screwdriver, while the other searched the house.
They stole cash and pension books.
The men then ran away along Coppull Moor Lane towards the railway bridge.
Police are investigating the aggravated burglary and are appealing for information to help them find the men responsible.
They said that although the family were not hurt in the incident, they were left “extremely shaken”.
Det Sgt Ian Derbyshire, of Lancashire Constabulary’s South CID, said: “This was an extremely nasty crime which has left the vulnerable victims very shaken up, although thankfully unhurt.
“I would appeal for anyone with information to get in touch with us.”
Police have released descriptions of the two men to help to identify them.
One of the intruders was described as male, about 6ft tall and of a slim build.
He was dressed in all dark clothing.
The second man was about 5ft 9ins tall and of a stocky build.
He wore a black cap and dark or black clothing
Anyone with information about the burglary is asked to call police on 101, quoting log number 1264 on Tuesday, November 18.
Alternatively, call independent charity Crimestoppers, anonymously, on 0800 555 111 or go online to Crimestoppers-uk.org.