Chicago Founding Member Walter Parazaider Dead at 81 Following Alzheimer's Diagnosis

Walter Parazaider -- one of the founding members of rock band Chicago -- is dead ... TMZ has learned.

His wife, JacLynn, tells TMZ ... Walter died at 2:10 AM Wednesday morning while in hospice, with her by his side. He was diagnosed with Alzheimer's 6 years ago and ultimately succumbed to the disease.

JacLynn adds ... "He had put up a good fight with Alzheimer's and unfortunately it ended tonight. We are going to miss him for sure ... We were married for 59 years and we had 59 wonderful years."

We're told there will be a memorial service at a later date.

Walter was best known for founding Chicago in 1967, though it was originally called The Big Thing before becoming Chicago Transit Authority and finally, Chicago, in 1969. He was one-third of the band's brass/woodwind section alongside Lee Loughnane and James Pankow.

He played with Chicago until he retired in 2016, contributing to hits such as "25 or 6 to 4," "Saturday in the Park," and "If You Leave Me Now."

He is survived by JacLynn and their daughters, Laura and Felicia.

He was 81.