Physical activity is essential for chronic disease prevention, yet <40% of overweight/obese adults meet the national activity recommendations. For time-efficient counseling, clinicians need a brief, easy-to-use tool that reliably and validly assesses a full range of activity levels, and, most importantly, is sensitive to clinically meaningful changes in activity. The Stanford Leisure-Time Activity Categorical Item (L-Cat) is a single item comprising six descriptive categories ranging from inactive to very active. This novel methodological approach assesses national activity recommendations as well as multiple clinically relevant categories below and above the recommendations, and incorporates critical methodological principles that enhance psychometrics (reliability, validity and sensitivity to change). We evaluated the L-Cat's psychometrics among 267 overweight/obese women who were asked to meet the national activity recommendations in a randomized behavioral weight-loss trial. The L-Cat had excellent test-retest reliability (?=0.64, P<0.001) and adequate concurrent criterion validity; each L-Cat category at 6 months was associated with 1059 more daily pedometer steps (95% CI 712-1407, ?=0.38, P<0.001) and 1.9% greater initial weight loss at 6 months (95% CI -2.4 to -1.3, ?=-0.38, P<0.001). Of interest, L-Cat categories differentiated from each other in a dose-response gradient for steps and weight loss (Ps<0.05) with excellent face validity. The L-Cat was sensitive to change in response to the trial's activity component. Women increased one L-Cat category at 6 months (M=1.0±1.4, P<0.001); 55.8% met the recommendations at 6 months whereas 20.6% did at baseline (P<0.001). Even among women not meeting the recommendations at both baseline and 6 months (n=106), women who moved ?1?L-Cat categories at 6 months lost more weight than those who did not (M=-4.6%, 95% CI -6.7 to -2.5, P<0.001). Given strong psychometrics, the L-Cat has timely potential for clinical use such as tracking activity changes via electronic medical records, especially among overweight/obese populations who are unable or unlikely to reach national recommendations.