Introduction:
Smoking habit in the youth is a new target of most tobacco companies. This habit can retard development and performance in athletes.

Objectives:
To find out the prevalence and effects of smoking in the IFMA Youth World Muaythai Championships, Bangkok 2018, we carried out as a short cohort study during the game.

Methods:
Seventy five candidates of the game with their ages ranging between 8 and 17 years, 70 males and 5 females, were interviewed and examined as the ordinary regulation. Furthermore, smoking and CO measurement from forceful expiration were also done in these candidates. They were followed up until the end of the game. Correlation among smoking, level of CO and the performance of the candidates were found out.

Results:
Forty five of seventy five or 60% of the candidates reported they did smoking. Twenty candidates, 26.6%, did smoking every day ranging from 1 to 15 times a day. CO measurement among the candidates revealed 1 to 9 ppm. On the other hand, CO measurement showed 0 to 1 in the nonsmoking candidates. Two of the candidates in non-smoking group received Gold Medal Award. In the candidates who could not pass the first round 28/32 or 87.5% were in the smoking group with CO in their force expiration over 5 ppm. Furthermore, none in the smoking group could pass the final contest.

Conclusion(s):
Prevalence of smoking habit in candidates of the IFMA Youth Muaythai Championships 2018 was very high. Smoking has negative effect on their performance of the game.