The direct measurement of true maximal oxygen uptake (VO_2max) and oxygen uptake corresponding to anaerobic threshold (VO_2AT) is not always practical, especially in middle age and older populations. Therefore, the purpose of this study was to develop a simple test that could accurately estimate cardiorespiratory fitness using a submaximal treadmill walking protocol for middle age, older, sedentary individuals and patients with chronic disease. Subjects for this study were 42 men (44.9 ± 15.7 years), which included 17 patients with coronary heart disease (57.0 ± 9.6 years). VO_2peak and VO_2AT were measured using a treadmill protocol (VO_2peak; 38.4 ± 11.6 ml/kg/min, VO_2AT; 22.9 ± 7.4 ml/kg/min). This simple test assessed the total distance covered in 12 minutes on the treadmill at an intensity corresponding to either 1) 11 on the Borg scale of ratings of perceived exertion (RPE11), 2) 13 on the Borg scale of ratings of perceived exertion (RPE13), or 3) “Optimal” by subjective judgment. The correlation coefficients between VO_2peak or VO_2AT and total distance at the three intensities (RPE11; 950 ± 100 m, RPE13; 1080 ± 140 m Optimal; 1050 ± 110 m) were statistically significant, ranging from 0.72 to 0.85. The test-retest reliability coefficient on 12 subjects was 0.98. The oxygen uptake (VO₂) was measured during the three walk tests on 15 subjects. There were no significant changes in submaximal VO₂ values from min 4 to min 12 (RPE11; 19.8 ± 4.7 ml/kg/min, RPE13; 24.1 ± 4.9 ml/kg/min, Optimal; 23.1 ± 4.8 ml/kg/min) in any of the three tests. Similarly, the three submaximal VO₂ values did not differ from the VO2AT value (21.2 ± 8.3 ml/kg/min) obtained in the initial maximal test. These results suggest that the 12-min submaximal treadmill walk test (STWT) is a valid method for the assessment of Voub>2peak and VO_2AT. Therefore, the STWT could be a useful performance test for evaluating cardiorespiratory fitness in middle age, older, sedentary individuals and patients with chronic disease.