The objectives of this research are to examine the factors affectingISO adoption in Thai public hospitals, and to test the theoretical modelwhich was derived from two organizational perspectives-institutional andstrategic choice. The integration of the two perspectives was used forexplaining determinants of this innovation adoption. Based on theintegration between these two perspectives, a conceptual model wasdeveloped in order to explain institutional factors, organizationalcharacteristics, strategic choice factors and leader characteristics asmain factors influencing ISO adoption in Thai public hospitals. Ninehypotheses were posited to test the model. Four of them were derived frominstitutional perspective, which explained the influence of socialpressure, policy directives, hospital age, and hospital size. Others werederived from strategic choice perspective, which explained relativeadvantage, compatibility, and complexity of ISO as well as leadercharacteristics such as their attitude toward change and their tenure. Research methods in this study were both quantitative and qualitativeanalyses. Mail surveys were conducted for 844 Thai public hospitals. Threehundred and forty three were received. In addition, 31 interviews wereconducted to support the results of the survey research. As well, 21department heads and 15 patients were interviewed for additional data forthis study. The quantitative study indicated that both institutional andstrategic choice factors were determinants of ISO adoption. The results ofthe model and hypotheses testing indicated that social pressure, hospitalsize, relative advantage, compatibility, technical complexity of ISO, aswell as leader tenure were major factors affecting the decision to adoptISO. The findings of the interviews revealed that hospital administratorsfelt that ISO was adopted primarily because of social pressure. Theythought that ISO adoption could increase prestige and recognition, whichwas one important reason a hospital would follow the example of another.Moreover, they indicated that free consultation, as well as attractivebudget were also factors leading to their adoption. The findings fromseveral interviews of hospital directors suggested the reason for ISOadoption was that it was a basis tool for improving service quality,boosting staff awareness and their morale, and also developing a qualityteam. These qualitative findings supported the survey that ISO adoption wasstrongly affected by social pressure apart from other factors thatrepresented rational decision-making. The implication for this research was that the theoretical modelderived from institutional and strategic choice perspectives should beapplied for explaining administrative innovations, because the innovationmay not be adopted strictly on a rational basis. It may be caused by otherinstitutional factors, particularly social pressure. It is recommended that administrators should be aware that socialpressure is a significant factor, but in itself should not be the solereason to adopt administrative innovation. Administrators need to carefullystudy any innovation before adoption to clearly determine that theinnovation can produce real organizational benefits.