Thirty-two patients with abdominal pain and/or intestinal pseudo-obstruction who had consumed raw or undercooked fish in the previous 72 h, were included in a study of anisakiasis, a parasitation of the human gastrointestinal tract by third stage Anisakis simplex larvae. Skin prick test (SPT) against A. simplex were positive in all the patients. High median eosinophil cationic protein (ECP) serum concentrations (> 15 mg/L) at day 0 with normal serum levels at day 30 and a rise of median total and specific IgE against A. simplex at day 30, were observed. We conclude that a raised serum level of ECP in the first 72 h from the onset of symptoms coinciding with a positive SPT against A. simplex and high total and specific immunoglobulin (IgE) in the first month after the parasitation, could be a useful tool in the diagnosis of gastrointestinal anisakiasis, even if the parasite cannot be isolated.