Hypertensive disorders which complicate 5-10% of all pregnancies are more frequent in diabetic women. We longitudinally monitored blood pressure (BP) for a 24 h period in 54 diabetic out patients, at each trimester of pregnancy, in order to observe the relationship between the blood pressure behaviour and the main clinical features of these subjects. A relationship was found between blood pressure and the diabetes type. In insulin-dependent diabetes mellitus (IDDM) women, the age of diagnosis and the diabetes duration played the major role whereas, in non insulin-dependent diabetes mellitus (NIDDM) women only the patients' age was correlated with the BP levels. Higher BP levels were found in women belonging to the White class D, and whose diabetes duration was more than 10 years. An interesting correlation was also demonstrated between BP and insulin requirement, in late pregnancy as well as the time of delivery. Early in pregnancy, BP levels, even if within normal range, were undoubtedly higher in those women who developed hypertension compared to those who continued to be normotensive. Daytime BP values were always found higher than night-time BP values but, the physiological biorhythm seemed to be altered at the end of pregnancy.