Sexually transmitted diseases (STD), whether old or new, impose a heavy economic burden on the public health service. Behavioural changes that could reduce the incidence of STD in the general population take time to make their effects felt. Common and treatable diseases such as genital chlamydial infections can and should be tackled by the old familiar strategy for combatting STD, viz. case finding, diagnosis, contact tracing and treatment, parallel with counselling in sexual behaviour. The incidence of infection is largely dependent on the number of asymptomatic individuals with a chlamydial infection in the population. Case finding programs devised chiefly for young, sexually active people will thus directly influence the natural life of Chlamydia trachomatis in our society. One such program, involving the sampling of women below the age of 30 when they come to the family planning clinic (FPC) for contraceptive advice, when applying for legal abortion, or when receiving prenatal care has been running in Halmstad, with increasing effect, since 1982. Contact tracing is an important part of the program. The prevalence of infected women has been halved since 1984 in the age interval 15-29 years. 10% of infected females, 15-24 years old, evidence repeated infection during 1 year. This program, carried out at a FPC, has proved cost-effective, achieving a 44% reduction in calculated expenditure. Chlamydial infections in Sweden are changing in character from being an untreated hyperendemic disease up to the late 1970s, to become a treated hyperendemic disease, until at last a declining prevalence can now be discerned.