This collection contains photocopies of the papers of Brigader General Adin B. Underwood. The papers include personal correspondence with transcriptions, Underwood's Union Army pension records, and a detailed history of the 33rd Massachusetts Infantry Regiment, Union Army, in which Underwood served.

Dating 1861 to 1865, the correspondence includes photocopies and transcriptions of letters written to General Underwood in Tennessee and Washington, D.C., by his wife Jane in Newtonville, Massachusetts, and his sister-in-law Maria, who resided in Brooklyn, New York. The letters written to Underwood pertain mostly to family matters and well wishes. Also in the correspondence are personal letters sent to Jane Underwood from her sister Maria, a Mrs. J. McQuesten, and an unknown sender regarding Adin's health and updates about friends and family. Lastly, the correspondence includes requests by General Underwood for leave of duty and the subsequent replies from his commanding officers, which are not transcribed.

The pension records span from 1861 to 1926. The records include photocopies of muster sheets for the 2nd and 33rd Massachusetts Infantry Regiments, USA, casualty and wounded reports, and pension forms and affidavits regarding Underwood. Orders from Adjutant General E.D. Townsend of the Union Army Headquarters request removal of Underwood's name from the 33rd Massachusett's regimental muster roll after he accepted his appointment to Brigader General. The pension records also contain forms completed by Underwood establishing his retirement pension. Lastly, forms and affidavits completed by Jane Underwood request her widow's pension following the death of Adin Underwood in 1888.

The Thirty-Third Massachusetts Infantry Regimental History details the recruitment and organization of the regiment in 1862. The document lists all forty-five battles and actions the regiment took part in for the duration of the war. The document also details the total wounded and killed as well as the mustering out and discharge of the surviving men of Thirty-Third after the end of the Civil War.