Informal payments refer to sums that patients may pay to individual or organizational health care providers outside of the official payment channels or approved fee schedules. The aim of the current research was to investigate informal payments and related influential factors in Urmia city hospitals. The present study was a cross-sectional survey conducted among post-discharged patients from all Urmia city hospitals during one Iranian calendar month (January 21 to March 19, 2013). Simple random sampling was used to recruit 265 patients to undergo assessment via phone call interviews and complete a questionnaire. Data analysis was performed using SPSS software for descriptive reports, and EViews software for determination of factors affecting informal payments. Eleven percent of the patients had made informal payments to physicians (mean amount: 503,000 Tomans, equivalent of $412), 5% to nurses (mean amount: 20,000 Tomans, equivalent of $16), and 17% to other employees (mean amount: 16,000 Tomans, equivalent of $13). Hospital ownership, patients' place of residence, education and income significantly influenced the payments. Most substantially, patients receiving surgical care were 100 times more likely to make informal payments compared to those who had received non-surgical inpatient care. The present study showed that although informal payment is illegal in Iran, it is a common practice among hospitalized patients, and has now become a challenge for the health system. Considering the high prevalence of informal payments and their severe impacts on equity and justice, policymakers have focused on this phenomenon to reduce and eliminate it.