The six sequential stool guaiac protocol has been advocated for screening of colonic cancer. Analysis of the expenditures involved in such a program shows that the cost of detecting cancer rises exponentially so that the marginal cost of the sixth test may be 20,000 times the average cost. The marginal cost is decreased with lower test sensitivity and increased with lower prevalence of colonic cancer. This result shows that even an inexpensive test can become quite costly in terms of cases detected. The marginal cost per case detected depends on the prevalence of the condition in the population screened and the sensitivity of the test applied.