One of the most preventable causes of mental and growth retardation is congenital hypothyroidism (CH). This study tries to investigate growth and mental outcome of patients with CH. Since November 2006 and November 2007 in Guilan province, north of Iran, all neonates who were diagnosed with CH, evaluated for etiology of CH by laboratory follow up, thyroid sonography or scan. Growth and development of patients with CH were compared with healthy children in same age, geographical area, social and economical classes in four years old. Demographic characteristics including height, weight, and head circumference at birth, follow up time (four years old) and IQ (Good enough test) were recorded in questionnaires. Among 28904 screened neonates, 37 patients with CH were diagnosed. Incidence of CH was 1:781 in live births, 20 (54%) in female neonates and 17 (46%) in male neonates. The incidences of permanent and transient hypothyroidism were 43.2% (16 cases) and 56. 8% (21 cases) respectively. The incidence of permanent and transient hypothyroidism were 16 (43.2%) and 21 (56, 8%), respectively. In permanent CH, 11 cases (%.68.2) had dyshormonogenesis and 5 cases (%.31.2) had thyroid dysgenesis. Significant statistical difference was only in family history of thyroid disease (34, 3% Positive family history in CH vs. 13.2% in control group, P-value 0.03). All other demographic characteristics and IQ had no statistical difference. Patients with CH diagnosed through neonatal screening and treated had normal growth as general population that indicates effective screening program and treatment in this area (3.2%).