The National Cancer Institute (NCI) has increased its emphasis in cancer prevention by undertaking a number of human intervention trials. Special emphasis is placed on chemoprevention, diet, and smoking and tobacco use. Some 24 clinical trials are ongoing in chemoprevention to assess the role of specific chemicals (natural and synthetic) in preventing, inhibiting, or reversing carcinogenesis. In diet and nutrition, several macronutrient trials are underway. The most significant of these is examining the relationship between dietary fat and breast cancer. In 1982, the NCI initiated a broad intervention research effort in smoking and tobacco use which has resulted in 46 prevention and clinical trials covering 25 states and more than 200 cities. More recently, a North American community-based intervention trial was begun to test cessation strategies for heavy smokers in 22 different sites. All of these intervention research efforts are testing strategies which later can be applied to large target population thereby supporting NCI's ambitious goal to reduce cancer mortality in this nation 50% by the year 2000.