Clinicians caring for sexually active adolescents are likely to be called on to diagnose and treat many of the STDs discussed in this article. A variety of other STDs not covered here also may be observed, including lymphogranuloma venereum, granuloma inguinale, molluscum contagiosum, scabies, pediculosis, and hepatitis A, B, and C. Some of the special issues related to gay and lesbian youth are discussed in the article by Drs Bidwell and Deisher (see "Adolescent Sexuality: Current Issues," pp 293-302). Nonetheless, it should be mentioned that the same STDs occur in homosexual youths as in their heterosexual counterparts. However, the prevalence rates for many STDs differ between the two groups, and some STDs are rarely seen in heterosexual males. These discrepancies may be explained by a number of determinants including anatomic and physiologic factors (eg, lesbian women have lower rates of STDs), differences in sexual practices (eg, genital-anal and oral-anal contact), and numbers of sexual partners, although this last factor may be less important in adolescents as compared with adult gay men. Discovery of one STD should always prompt a search for others because multiple concurrent infections is the rule rather than the exception. A serologic test for syphilis and a discussion of the potential for HIV infection (possibly testing for HIV as well) should take place at each new encounter for an STD. Some patients, including those with multiple partners, have an increased chance for acquisition of an STD. However, the reality is that any adolescent who has had sexual intercourse could have an STD.(ABSTRACT TRUNCATED AT 250 WORDS)