Systolic blood pressure was measured in 98 very low birthweight (VLBW) infants in the first 48 h of life. Measurements were made on 49 male infants, median gestational age 29 weeks and 49 female infants who were matched with the male infants for gestational age. Blood pressure was measured either from an indwelling arterial line or non-invasively using a Doppler technique; measurements were made on all infants on day 1 and day 2. In both male and female infants on day 1 and day 2, blood pressure levels correlated significantly with gestational age (P < 0.01). On day 1, but not on day 2, the blood pressure of the male infants was significantly lower than the female infants. We conclude it is important to use gender appropriate regression equations for blood pressure in VLBW infants in the first 24 hours of life.