The objective of this study was to evaluate the effect of central, obstructive and mixed apnea on cerebral total hemoglobin concentration (tHb), which is analogous to cerebral blood volume, and to investigate whether tHb alterations correlate with bradycardia and arterial desaturation. Measurements were carried out on 17 preterm infants (gestational age 26-30 weeks) with frequent apneic events. Near infrared spectrophotometry (NIRS) was used to quantify changes in tHb. Respiration was monitored by chest movements using impedance pneumography and by nasal airflow using a thermistor. In addition, heart rate, arterial oxygen saturation, in each infant and esophageal pressure in 3 babies were continuously recorded. 130 apneic episodes of > 10 s duration showed four different patterns of tHb alterations: (1) no change in tHb (28%); (2) isolated decrease (35%); (3) isolated increase (12%), or (4) both combined, an initial decrease followed by an increase over the previous baseline level (25%). Obstructive apneic episodes were associated with a significantly greater maximum fall in tHb (median 11.5; 5th percentile 0 and 95th percentile 30.5 mumol/l) compared to mixed (4.9, 0 and 26.4 mumol/l) and central events (3.0, 0 and 14.0 mumol/l). Changes in tHb correlated with heart rate only in purely central apnea and were not reflected in arterial oxygen saturation in any type of apnea. Obstructive apnea was observed to have the strongest impact on tHb. As these tHb alterations may exacerbate or cause intraventricular hemorrhage, efforts must be made to prevent obstruction of upper airways and to focus monitoring on cerebral perfusion.