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Amblyopia screening effectiveness at 3-4 years old: a cohort study. Objective: To study the effectiveness of amblyopia screening at ages 3-4. Methods and Analysis: From a population with no previous screening, a cohort of 2300 children with 3-4 years old attending school (91% of children this age attend school in Portugal), were submitted to a complete ophthalmological evaluation. Amblyopia was diagnosed, treated and followed. Amblyopia prevalence, treatment effectiveness, absolute risk reduction (ARR), number needed to screen (NNS) and relative risk reduction (RRR) were estimated. Results: Past/present history of amblyopia was higher than 3.1%-4.2%, depending on amblyopia definition normatives. Screening at age 3-4, had estimated ARR=2.09% (95% CI 1.50% to 2.68%) with a reduced risk of amblyopia in adulthood of 87% (RRR). NNS was 47.8 (95% CI 37.3 to 66.7). Treatment effectiveness of new diagnosis was 88% (83% if we include children already followed). 91% of new amblyopia diagnoses were refractive (of which 100% surpassed amblyopia Multi-Ethnic Pediatric Eye Disease Study criteria after treatment), while most strabismic amblyopias were already treated or undertreatment. Only 30% of children with refractive amblyopia risk factors that were not followed by an ophthalmologist, ended up having amblyopia at age 3-4. Eye patch was needed equally in new-diagnosis versus treated-earlier refractive amblyopia. Conclusions: Screening amblyopia in a whole-population setting at age 3-4 is highly effective. For each 48 children screened at age 3-4, one amblyopia is estimated to be prevented in the future (NNS). Screening earlier may lead to overdiagnosis and overtreatments: Treating all new diagnosis before age 3-4 would have a maximal difference in ARR of 0.3%, with the possible burden of as much as 70% children being unnecessary treated before age 3-4.Involving primary care, with policies for timely referral of suspicious/high-risk preverbal children, plus whole screening at age 3-4 seems a rational/effective way of controlling amblyopia.