The false negative rate of the stabilized guaiac faecal occult blood tests, Fecatest and Haemoccult, was determined in 61 patients with known colorectal cancer. When subjects tested their stools over three days, 9.7% of the Fecatest slides and 27.8% of the Haemoccult slides were negative (p less than 0.02). Extending the number of days of testing from three to six reduced the false negative rate of Fecatest to 4.8% and Haemoccult to 9.7% (p less than 0.05). There was no significant difference between the false negative rate of Fecatest and Haemoccult test when subjects performed six-day testing. Haemoccult false negative results occurred mainly in patients with well differentiated left sided carcinomas.