This study investigated the Cause-Effect of Five Child-Killer Diseases on Under-Five Mortality (Deaths) in Adamawa State, Nigeria. The study used Ex post facto and quantitative design. A secondary data was obtained from Adamawa State Primary Health Care Development Agency (PHCDA), Yola across the study area for the period of 2001 to 2020.The study used Multiple Regression Analysis to determine the Cause-effect of the Five Child-killer Diseases on Under-Five Mortality in the study area. The study revealed that; Polio needs to be excluded from the Five Child-Killer Disease since there is no death associated with polio disease for the period under review in the study area. The results also revealed that the deaths due to diarrhoea have a significant causal relationship on Under-Five Mortality, while deaths due pneumonia, measles and tetanus has insignificant causal relationship on Under-Five Mortality in the study area. Based on the overall model, the Five Child-Killer Diseases have a Cause-effect of 64% on Under-Five Mortality in the study area. In view of the above, the study recommends that Adamawa State Primary Health Care Development Agency, Yola whose primary responsibility among orders is to curb and reduce the spread of Child-Killer Diseases among vulnerable children of ages; needs to review and strategies their action plans, since the Cause-effect is still high.