The purposes of this study were to establish a Computer Assisted Instruction of "THYROID FUNCTION & DISEASES" and to compare pre-test and post test scores as well as to compare the opinions on using Computer Assisted Instruction done by medical students. The investigator constructed this Computer Assisted Instruction. The sample group was composed of 18 fifth year medical students who were in the first semester of the 1992 academic year at Faculty of Medicine Siriraj Hospital, Mahidol University. They were selected by Grade Point Average of 2.5 to 3.0 for the 1991 academic year and then were devided in to three groups of 6 students each. The experimental group I was studied by listening to a lecture, followed by using the Computer Assisted Instruction, group II by using the Computer Assisted Instructions alone, and group III by reading textbook by themselves respectively. The score of three groups were tested before and after their study immediately. Consequently, except group III, the students completed a questionnaire about their opinions on using the Computer Assisted Instruction. Mean, Standard deviation, The Kruskall-Wallis One-Way Analysis of Variancc by Ranks and The Mann-Whitncy U Test were applied to analyze the data. The Results were as follows: 1. There was no significant difference of the comparative students of group I and 11 at 0.05 in pre-test scores, post-test scores and opinions on using the Computer Assisted Instructions. That would mean that the Computer Assisted Instruction in use effectively complemented the traditional teaching model for medical students. Most of them showed positive response to the C.A.I. 2. The mean scores of the experimental group I and II were higher than those of group 111. These results suggested that studying with the assistance of the C.A.I. had a trend towards more effectiveness than just reading the book by oneself.