Objective: To determine the frequency & associated factors of premenstrual syndrome in medical collage students. Methods: A cross sectional study by random sampling was conducted during July to September 2009 among the 200 medical students aged b/w 18-25 years of private medical institution. Questionnaire was derived from DMS-IVdiagnostic criteria for premenstrual dysphonic disease and PMS symptoms. Results: All of the students (100%) who participated in this study reported some degree of at least one symptom. Using DSM-IV diagnostic criteria prevalence of PMS was found to be 59%. The order of frequency of psychological symptoms include increase appetite 67.5%, worry and anxiety 60%, tired or lethargic 54%, felt suddenly sad / tearful 56.5%, interpersonal conflict 54% and depressed mood 52.5% and physical symptoms were found in the order of backache 55%, acne 50.5%, joint or muscle pains 49.5%, abdominal bloating 44.5% and weight gain 40.5%. Age (p =.460), Living condition (p=0.270) and residence (p=0.170) showed no association with the severity of symptoms of PMS. Conclusion: PMS is frequently occurring problem in medical students affecting the mental well being and concentration of students on their academics. It requires urgent identification and attention for better academic achievements.