Background: There is no a comprehensive study on early-onset multiple sclerosis (EOMS) population on the role of well-known MS risk factors. Moreover, age at puberty and the critical window exposure to environmental risk factors is never investigated in neither adult nor pediatric MS population.Methods: A case-control study, including 97 patients with early-onset MS and 97 matched controls, was conducted in Isfahan Multiple Sclerosis Society. Information on age at puberty, gestational risk factors, environmental exposures, and past medical history was obtained from medical charts and interviews.Findings: In logistic regression model, every year of decreasing in pubertal onset in females and males was associated with 1.766 and 2.250 folds increase in risk of early-onset MS, respectively. In utero exposure to cigarette smoking was significantly associated with 2.33 (P = 0.010), and having smoker parents or passive smokers before puberty had 2.88 (P =0.006) fold in risk of early-onset MS. One hour less daily sun exposure increased the risk of early-onset MS as 2.007 folds (P < 0.001) after adjustment for age and sex.Conclusion: In utero and parental exposure to smoking as well as history of measles and appendectomy are risk factors for early-onset MS. Future studies should shed more light on the true interaction between pineal gland, gender, smoking, sunlight and other environmental and genetic risk factors in adult and pediatric MS population.