Precise diagnosis of peri-intraventricular hemorrhage (PIVH) requires brain real-time ultrasound imaging procedure (US). However, maximal diagnostic efficiency of US lies between day 4 and 14 since fresh blood may initially appear sonolucent. Because of this supposed interval required for clot formation to become visible on US, serum CPK-BB estimations were performed in the first 60 hours of life to determine precise biochemical timing of PIVH. A group of 50 preterm infants less than 1500 g birth weight (1120 +/- 320 g) and 34 weeks gestation (30 +/- 3.7 weeks) was studied. Serial CPK-BB measurements were performed in serum immediately after birth (T0), then serially at time T1 (6-10 h), T2 (20-30 h), T3 (40-60 h). The incidence of PIVH diagnosed on the third day of life was 30%. Total CPK-BB values at T0 in infants who developed PIVH were significantly higher than those of patients without cerebral bleeding (70.8 +/- 30.5 vs 20.9 +/- 10.7 U/l) (p less than 0.05). The same statistically significant results were not observed analysing the CPK-BB values at T1, T2 and T3. These results suggest that most pathological conditions responsible for enzyme release occur in the pre- or perinatal period.