 Therefore, the aim of the present study was to follow-up and examine after 3.5 years, the efficacy of sealing caries-free or non-cavitated mesial surfaces of first permanent molars abutting lesions on the distal surfaces of second primary molars. The null hypothesis tested here was that preventive and therapeutic sealants do not prevent the development or slow the progression of dental caries over a period of 3.5 years in comparison to non-sealed control surfaces. 