Thrombocytopaenia occurs during necrotizing enterocolitis (NEC), and nadir platelet count is associated with extent of disease. In paediatric meningococcal disease, the product of neutrophil and platelet count at admission is prognostically useful. We therefore aimed to determine whether the first full blood count (FBC) after diagnosis of NEC is useful as a score for poor outcome and severity of disease. Between 1987 to 2001, neutrophils (N), platelets (P) and their product (PN) was available in 187 neonates treated for NEC at our institution. Neonates with NEC were grouped according to the extent of disease (no gangrene, focal, multifocal and pan-intestinal). Data were not normally distributed so Mann-Whitney U test or analysis of variance (ANOVA) on logged data were used ( p<0.05 was considered significant). Receiver operating characteristics (ROC) curves were used to examine the relationship between specificity and sensitivity. A perfect test would have an ROC curve area of 1. Initial P count and PN product of non-survivors were both significantly lower than in survivors ( p<0.0001), whereas N was not different ( p<0.08). Low Log(10)PN was significantly associated with greater extent of disease (ANOVA; no gangrene vs multifocal, p<0.01, vs panintestinal, p<0.0005), suggesting that the initial FBC could be prognostically useful. Area under the ROC survival curve for neutrophils was 0.58, for platelets 0.75 and for PN product 0.71; thus, although no test performed extremely well, initial platelet count and NP product could be useful in evaluating disease severity in neonates with NEC and for further monitoring.