The Communist governments of Hungary and Yugoslavia prevented Cardinals József Mindszenty and Aloysius Stepinac from traveling to Rome.

The conservative, supporting Vatican centralization of authority, Giuseppe Siri of Genoa was only 52 and his election would have meant another long papacy like that of Pius IX.

Angelo Giuseppe Roncalli was now the Patriarch of Venice after more than 25 years in the diplomatic service of the Holy See in Bulgaria, Turkey and France.

Approaching 77, his age marked him as a possible compromise choice in expectation of a short pontificate, along with his "reputation for being broad-minded and conciliatory".

Benedetto Masella, the 79-year-old chosen as camerlengo on 9 October,[1] a veteran diplomat, was also mentioned as a compromise candidate with "his chances diminished because of his age".

[12] Also mentioned as a radical departure from tradition was Giovanni Battista Montini, Archbishop of Milan, whom Pius had not made a cardinal.

[8] The New York Times cast a wide net, offering more than a dozen names, including two non-Italians, Paul-Émile Léger of Montreal and Manuel Goncalves Cerejeira of Lisbon.

[8] Life magazine's coverage included portraits of Agagianian, Lercaro, Montini, Ottaviani, Roncalli, Ruffini, Siri, and Valeri.

[14] A Moscow radio broadcast criticized Pius for meddling in politics and hoped for a new pope devoted instead to "religious problems".

[16] By the second day of the conclave, after four ballots produced no results, speculation centered on Roncalli, Valeri, Masella, and Agagianian, the first three elderly and the last an unlikely outsider.

[17] When one cardinal-designate, Carlo Agostini, died on 28 December at the age of 64,[18] the Vatican announced another cardinal designate the next day, Valerian Gracias of India, so the College reached its full complement of 70 members, with 26 of them Italian.

[30] Giovanni Battista Montini (later Pope Paul VI), a longtime curia official who had recently become Archbishop of Milan, consistently received two votes even though he was not yet a cardinal.

Requests from a doctor inside the conclave for medical records suggested several cardinals were ill.[32] It took a few ballots for supporters of Lercaro, "who was known to favor a simplified liturgy in local languages", and "the aggressively sententious" Siri to recognize they could not garner the necessary 35 votes.

[41] Following an old tradition, immediately after his election, Pope John gave his scarlet zucchetto to the Secretary of the conclave, Alberto di Jorio.

The conclave ended the next day after Mass in the Sistine Chapel and an address by Pope John to the cardinals which was broadcast on radio.

"[47] In early November, Pope John wrote letters to Mindszenty and Stepinac expressing regret that they were unable to participate in the conclave.

[51] John XXIII waited several years before issuing a motu proprio to modify certain aspects of the procedures for a papal conclave.

In Summi Ponitificis electio, issued on 5 September 1962, he laid out additional rules for impressing all participants with the need for secrecy, even warning the cardinals about communications with their staff (paragraph XIV).