Decompression illness (DCI) is attributed to the formation of bubbles, resulting from the reduction of the ambient pressure. Circulating bubbles lead to capillary leak syndrome, extravasation of plasma and haemoconcentration. Experimental model on animals has shown that a haemoconcentration carried a poor prognosis. We measured the haematocrit level in fifty-eight consecutive sport divers, victims of neurological DCI, admitted to our hyperbaric center, and in sixteen control divers. No significant difference was found in the haematocrit values between the divers with neurological DCI (median 42.5%) and the controls (median 41.75%). The median haematocrit level was significantly higher for divers with neurological sequelae when compared with control (p = 0.01) or with divers without sequelae (p < 0.05). A haematocrit level > or = 48% was correlated with persistent neurological sequelae one month after the accident (p = 0.01). However, a haematocrit < 48% had no prognostic value.