Combined antiretroviral therapy has reduced both AIDS mortality and morbidity. An unknown proportion of patients is identified early and starts therapy before developing AIDS, thus escaping epidemiological surveillance. For this reason it is important to monitor the trend of new diagnoses of HIV infection. From the comparison of patients living in the Province of Sassari with new diagnoses of HIV infection or AIDS in the period 1997-2003 some differences emerge. Males are the most affected, but the difference tends to decrease among new HIV cases. Sexual contact is the most common route of transmission among new HIV diagnoses, whereas the parenteral route prevails among AIDS cases. An increase in the percentage of foreigners has been found only among new HIV cases. The difference found between new AIDS and HIV cases emphasises the importance to implement HIV infection based surveillance systems, in order to better guide informative campaigns and other interventions.