To determine demographic, epidemiological, clinical features and risk factors of sexually-transmitted diseases (STD) in adolescents and young adults referring to a STD centre in Northern Italy, patients diagnosed with a STD and aged 13-20 years, were prospectively evaluated from the year 1991. Teenagers showed an appreciable and increasing risk of a broad spectrum of STD, and represented 3.6-13.3% of patients yearly diagnosed with a STD in a nine-year period. More than 10% of patients were under the age of 18, and did not report condom use in the six months preceding diagnosis in 96.7% of cases. A significant increase in the female gender was found since 1993 (but our series was massively influenced by the recent immigration of female sex workers, who declared frequent condom use), while male homosexuals and drug addicts represented a minority. When excluding subjects engaged in prostitution, over 50% of patients declared only 0-1 sexual partners during the last six months, but a significant increase in sexual promiscuity was observed over time (1997-1999 versus 1991-1996) (p<.02). Over two thirds of our teenagers never used a condom prior to the diagnosis of STD, and the apparently increased temporal trend to condom utilization proved related only to the recent immigration of sex workers; lack of condom use proved significantly related to the male gender, and a lower education. Non-gonococcal STD and human Papillomavirus infection accounted for over 60% of overall diseases, while the frequency of molluscum contagiosum, gonorrhea, HSV genital ulcer, and Chlamydia disease ranged from 6.1 to 7.4%, and that of syphilis, phthiriasis, trichomoniasis, and HIV infection varied from 1.3 to 4.7%. Non-gonococcal STD and syphilis were increasingly diagnosed over time, especially in immigrant prostitutes (and despite their frequent condom use). According to our surveillance study of teenagers, a number of demographic, epidemiological, and clinical features of STD showed significant variations over time: recent immigration, spread of prostitution, increased sexual promiscuity, and infrequent condom use, are relevant and/or persisting risk factors. A permanent monitoring of STD in young adults and adolescents is strongly warranted, to allow a timely diagnosis and an appropriate treatment, and to plan preventive strategies specifically addressed to this target population, with special attention to immigrants and sex workers.