Underwood laid the foundation of modern pediatrics. His work was superior to anything that had previously appeared and remained the most important book on the subject for sixty years, passing through many editions. The first edition (p.76) includes the first description of sclerema neonatorum (“Underwood’s disease”). That edition also contains a description of "aphthae of thrush."

In the second edition (1789, volume 2, pp. 122-27) Underwood presented a description of congenital heart disease in children. This was the first pediatric treatise to do so. Also, in the second edition, volume 2, pp. 53-57 entitled "Debility of the lower extremities," Underwood was the first to consider poliomyelitis as an entity.