Orthodox Christians venerate John as a wonderworker. The following are attributed testimonies, not independently verified medical findings or promises that prayer will produce a particular outcome.
Sergei Bogatsky's recovery
In a published remembrance, Sergei Bogatsky recalled a serious lung illness and John's hospital visits. After medication failed, surgeons removed part of his left lung. He remembered waking after the operation to find John beside him, holding his hand. Bogatsky attributed his successful surgery and lasting recovery to John's prayers. His testimony includes medical treatment; it should not be retold as a recovery without surgery. Source: Sergei Bogatsky, remembrance 7, pages 14–15.
A healing reported at the relics
Archpriest Peter Perekrestov recalled a sick boy being brought to John's relics during their opening in 1993. He reported that the boy was healed after touching them and later enjoyed good health. The account does not provide a diagnosis or clinical records, so those details are not supplied here. Source: Perekrestov, Opening of the Relics, and Glorification of St. John Maximovitch.