BACKGROUND: Butere-Mumias district is one of the districts which performs poorly on immunisation coverage, as observed from the Kenya Expanded Programme of Immunisation reports. OBJECTIVE: To identify factors that contribute to the low level of immunisation coverage in the district among children under the age of five years. DESIGN: Cross-sectional descriptive study. SETTING: Butere-Mumias district. RESULTS: Out of the 293 mothers who were sampled, 238 (80%) had attained primary level of education while, 55 (20%) attained secondary level and above. Immunisation coverage was found to be 35% in this district. Long distances to health facilities, poor states of the roads, age, attitude and knowledge regarding immunisation among mothers were significant factors that determined immunisation coverage. CONCLUSION: Low levels of education, long distances to the nearest health facilities, lack of knowledge on immunisations and lack of staff were responsible for the low coverage. There is therefore, need to act on these factors which hamper the immunisation coverage in order to reduce child mortality rate which are attributable to non-immunisation.