The aim of this study was to determine long-term efficacy and safety subcutaneous injection of interferon-alpha-2B in patients with mumps orchitis in terms of testicular volume and other testicular functions. Mumps orchitis was evaluated in 37 patients. Patients were hospitalized and administered 1 × 3,000,000 IU subcutaneous injection of interferon-alpha-2B daily for 7 days. The testicular volumes of all the patients were measured by ultrasonography in the 18th month following treatment termination. The testes volumes were evaluated by descriptive statistics as percentages. Patients were divided into three groups according to testes volumes and differences between the involved and non-involved testicles. Group I included patients with normal testes volume (> 12 ml) and a difference between testes of less than 2 ml or 20%; Group II (atrophic groups) included patients with testes volume of less than 12 ml; and Group III (hypotrophic groups) included patients with testes volume of greater than 12 ml and a difference between testes of more than 2 ml or 20%. Groups were compared in terms of results of semen analysis and serum follicle stimulating hormone (FSH) and luteinizing hormone (LH) levels. Patients' ages ranged between 17 and 41 years (mean: 28.3 years). A total of nine atrophy cases were identified. Sixteen patients were determined to have hypotrophic testes with a difference of 2-10 ml or 20% between the involved and non-involved testicles, despite the absence of testicular atrophy. A comparison of groups revealed that sperm density, total sperm count, total motile sperm count, and motility percentage were significantly higher in Group I than in the other groups, while serum FSH and LH levels were lower in Group I than in the other groups. Although the use of interferon-alpha-2B appears to prevent testicular atrophy and protect testicular function, it leads to a considerable difference in the volume between testicles and a significant loss of testicular function. As a result, further studies are required in order to investigate the efficacy of the drug.