The Stornoway ferry, MV Loch Seaforth, had to turn back to Ullapool after a medical emergency on board on Saturday night.
The vessel was heading past the Summer Isles into the open Minch when she turned around.
It is understood a male traveller suffered a serious asthma attack.
The ship briefly docked at Ullapool where an ambulance was waiting at the quayside.
A spokesman for CalMac said: “Due to a medical emergency on board the MV Loch Seaforth, the sailing had to return to Ullapool.”