Asthma is the major chronic respiratory inflammatory disease in all ages. The validity and predictability of revised Jones Morbidity Index (JMI) as a simple and practical tool for asthma evaluation morbidity has not yet been tested in Iran. This study aimed to evaluate the predictability and validity of a revised JMI in northern Iran. As a one-year follow up study, 210 adult asthmatic patients were visited in an asthma clinic in the town of Babol (north of Iran) at two stages. At first stage, in addition to demographic information, 3 simple questions of revised JMI were asked and Pulmonary Expiratory Flow Rate (PEFR) for each patient was measured. Based on modified JMI, patients were categorized in three groups: Low, Medium, and High morbidity. At the second stage, after one year, patients were visited again and in addition to their last year medical records, the same questionnaires were filled. The validity of the index checked by PEFR values at two stages and its predictability was evaluated by morbidity factors during the last year. From hundred and seventy (170) patients who were able to fulfil the second stage of the study, 55 patients (32%) had been categorized as low, 88 patients (52%) as medium, and 27 patients (16%) as high morbidity. The percentages of patients PEFR to the estimated normal values in these three categories (90.8%, 74.7%, and 55.3% respectively) were significantly different which shows a good correlation between PEFR values and asthma morbidity (p<0.001). The relative risks of high morbidity group for hospitalization from asthma and needing oral steroids during the one year period were 4.1 (CI = 1.27 to 13.1), 1.96 (CI = 0.97 to 3.96) respectively which are significantly higher than the other two categories (P<0.05). Since the modified JMI showed an acceptable validity and predictability in northern patients of northern Iran, we recommend its use as a simple pragmatic tool for evaluating asthma morbidity in primary care in this region of Iran.