Our objective was to study the relationship between dispensed aspirin, nonaspirin nonsteroidal antiinflammatory drugs (NSAIDs), steroidal antiinflammatory drugs (SAIDs), acetaminophen, calcium, psyllium, and multivitamin preparations and the risk for subsequent colorectal adenoma and adenocarcinoma. The design was a case-control study. The patient population was from a large municipal teaching hospital in Atlanta, Georgia. In logistic regression models, the risk of colorectal adenoma or adenocarcinoma decreased in the first two years of continuous NSAID use in a linear, time-dependent manner. The risk of colorectal neoplasia after two years of continuous NSAID use was reduced significantly (P < 0.01) as compared to nonusers. Risk reduction appeared greater for adenocarcinoma than adenoma. The use of SAIDs, calcium, multivitamins, and psyllium, as prescribed to our patient population during the mean six-year study period, conferred no measurable risk reduction. These results suggest that in prospective chemoprevention trials, a significant risk reduction can be expected after only two years of aspirin use, in doses similar to those recommended for the prevention of cardiovascular disease, or nonaspirin. NSAIDs, in doses commonly prescribed for the management of musculoskeletal pain. The results also imply that any short-term reduction in the incidence of colorectal adenoma detected in a phase II trial would underestimate the chemopreventive effect of NSAIDs on the risk of adenocarcinoma.