Objective: We examined the association between the number, magnitude, and frequency of febrile episodes during the 0 to 4 years of life and subsequent diagnosis of ADHD. Methods: This population-based case-control study in an Israeli HMO, Leumit Health Services (LHS), uses a database for all LHS members aged 5 to 18 years between 1/1/2002 and 1/30/2022. The number and magnitude of measured fever episodes during the 0 to 4 years were recorded in individuals with ADHD (N = 18,558) and individually matched non-ADHD controls in a 1:2 ratio (N = 37,116). Results: A significant, independent association was found between the number and magnitude of febrile episodes during the 0 to 4 years and the probability of a later diagnosis of ADHD. Children who never had a measured temperature >37.5°C had a significantly lower rate of ADHD (OR = 0.834, 95% CI [0.802, 0.866], p < 0.0001). Conclusions: Febrile episodes during 0 to 4 years are associated with a significantly increased rate of a later diagnosis of ADHD in a dose-response relationship.