The World Health Organization (WHO) is about to publish the 11th revision of the International Classification of Diseases and Related Disorders (ICD-11). A pre-final version has been disseminated in 2018, in order to allow the 194 member States to provide suggestions for amendments and to prepare shared strategies for the implementation of the diagnostic system. Furthermore, the Ecological Implementation Field Studies (EIFS) have been conducted in various countries, with the aim to assess the reliability and clinical utility of the ICD-11 diagnostic guidelines. In this paper we present the overall methodology of the EIFS and discuss the results of the Italian field trials concerning the reliability in the use of ICD-11 diagnostic guidelines concerning schizophrenia and related disorders. In Italy the field trials have been carried out at the Department of Psychiatry of the University of Campania "L. Vanvitelli", WHO Collaborating Centre. The inter-rater reliability of the diagnostic guidelines for schizophrenia and related disorders has been assessed on the basis of clinical interviews with 100 patients conducted by 14 psychiatrists. The results show an almost perfect reliability for the diagnosis of schizophrenia (kappa=0.85) and a good reliability for the diagnosis of schizoaffective disorder (kappa=0.79). These Italian data are in line with those of the international sample, and the kappa values are higher than those previously reported in the ICD-10 field trials. They suggest the feasibility of the implementation of ICD-11 diagnostic guidelines in ordinary clinical practice in our country.