Background: Immunization is today one of the safest, most cost-effective, and powerful means of preventing deaths and improving lives. Even though immunization currently averts an estimated 2 -3 million deaths every year, an estimated 21.8 million infants worldwide are still missing out on basic vaccines. Missed opportunities for immunization contributes to the low immunization coverage seen worldwide adding to the pool of children who have not been effectively immunized. These pockets of low coverage create a risk for the reintroduction of vaccine-preventable diseases (VPDs) that have been eradicated, eliminated, or are under epidemiological control. The Western region of Kenya has a long history of poor vaccine coverage. The devolution of healthcare to the County provides an opportunity for a more detailed understanding of the challenges and barriers in the local setting. This study was conducted in Siaya, one of the counties in the Western region of Kenya. Objective: To evaluate the prevalence and factors associated with missed opportunities for immunization among children aged below 60 months at Siaya County referral hospital. Methodology: This was a hospital based descriptive cross sectional study of children seeking health services at Siaya County referral hospital. The study was conducted between January and February 2016 at the Paediatric inpatient and outpatient departments at the facility. The study groups included eligible parent/guardian-child pairs and eligible health care workers who had consented to participate in the survey. A total of 370 child-parent/guardian pairs were interviewed.