To compare the incidence of cough between two angiotensin converting enzyme (ACE) inhibitors, imidapril and enalapril, comparative crossover study was performed in 489 patients (228 men and 261 females) with essential or renal parenchymal hypertension. Patients were randomly assigned to one of two treatment groups, a group receiving imidapril for 12 wk (Period I) followed by enalapril for 12 wk (Period II), and a group in which the order of drugs was reversed. The occurrence of cough during treatment was monitored by questionnaire in all cases. There were no differences in background characteristics between the two groups. The incidence of cough during Period I was 15.2% (32/210) in the group initially treated with imidapril (Group IE) and 38.6% (85/220) in the group initially treated with enalapril (Group EI), the difference being statistically significant (p < 0.001). During Period I, decrease in blood pressure was observed in 63.9% (115/180) of Group IE and 64.6% (115/178) of Group EI patients. In approximately half of the patients in Group EI who developed cough during Period I and in whom the treatment was subsequently switched to imidapril, cough subsequently disappeared. It was concluded that the incidence of cough was significantly less under imidapril than under enalapril treatment, while there was no difference in the antihypertensive effects of the two ACE inhibitors.