I MET her at 2 a.m. on a cold and windy morning in Washington, when she ran over to the outreach van to get a warm cup of coffee. Volunteers were offering condoms and health information to sex workers. She took only two condoms, and I urged her to take more. She told me that although she was worried about H.I.V., she was more afraid of the police. A month earlier, she had been harassed by officers for carrying several condoms. They told her to throw them out. She thought if they picked her up with more than a couple of condoms again, she might be taken to jail on prostitution charges.

Her story is not unique. Over the last eight months, Human Rights Watch has interviewed more than 200 current and former sex workers in New York, Los Angeles, Washington and San Francisco. The interviews were part of an investigation into barriers to H.I.V. prevention for sex workers, who, worldwide, are more than 10 times as likely to be infected as the general population. What we found was shocking: While public health departments spend millions of dollars promoting and distributing condoms, police departments are harassing sex workers for carrying them and using them as evidence to support arrests.

Many of the women we interviewed asked, “How many condoms is it legal to carry?” One wondered, “Why is the city giving me condoms when I can’t carry them without going to jail?” Some women said they continued to carry condoms despite the consequences. For others, fear of arrest trumped fears of sexually transmitted diseases and pregnancy. Most of those we interviewed told us they were afraid to carry the number of condoms they needed, and some — about 5 percent — told us they had unprotected sex with clients as a result.

Police officers confiscate condoms and prosecutors try to enter them as evidence not because it is official policy to do so, but simply because they have not been trained to do otherwise. An act of the legislature (like one bill pending in the New York State Assembly), or even a directive from a police chief or district attorney, could end the practice immediately. Categories of evidence — like testimony regarding the sexual history of rape victims — are excluded as a matter of public policy in many legal systems. In this case, the value of condoms for H.I.V. and disease prevention far outweighs any utility they might have in the enforcement of anti-prostitution laws. Law enforcement efforts should not interfere with the right of anyone, including sex workers, to protect his or her own health.