A Louth man was found unconscious in Queen Street having suffered head injuries on Saturday morning.
The 44-year-old man was found on the ground close to the junction with Burnt Hill Lane at 2.14am and was rushed to Grimsby Hospital before being transferred to Hull Royal Infirmary.
He is in a serious condition with a suspected bleed on the brain, but is currently stable.
Police do not know how he came by his injuries, he may have fallen or alternatively been involved in some kind of incident.
The investigating officers DC Nick Jones and DC Rhys Tallentire of Louth CID are appealing for information and are asking for anyone who witnessed what happened to contact them via the non-emergency 101 number quoting incident number 46 of July 5.