The magnitude and duration of the effect of dexamethasone on systolic blood pressure has been examined in 13 very preterm infants (median gestational age 25 weeks). All had chronic lung disease (CLD). To exclude any effect of CLD on blood pressure each infant acted as his or her own control. Systolic blood pressure increased in all infants (P less than 0.01) and remained elevated for at least 48h following cessation of therapy. The median maximum increase in blood pressure was 24 mmHg (range 13-49 mmHg) and occurred on day 4 (median, range 2-10) of treatment. One infant developed hypertensive encephalopathy. These results demonstrate the need to monitor infants with CLD throughout steroid therapy and preferably for some days after it has ceased.