Background <p>Accurate assessment of physical activity behaviors allows clinicians to target counseling more effectively. Single questions regarding habitual physical activity provide the most feasible option for rapid physical activity assessment. This study evaluated construct validity of a single item physical activity question to self-report habitual activity as simple categorical descriptors (<i>not active</i>,<i> somewhat active</i>,<i> active</i>,<i> very active</i>).</p> Methods <p>Women (<i>N</i> = 120) completed one laboratory session for measurement of self-reported physical activity (International Physical Activity Questionnaire-IPAQ), anthropometrics and body composition, and physical activity-related quality of life (SF-36 physical function subscale, Vitality Plus Scale). Construct validity was evaluated as convergent and known-groups validity.</p> Results <p>Mean age was 60 ± 16 years (range 25–89). The majority (62%) reported being <i>active</i> or <i>very active.</i> Age was not significantly related to activity levels. Correlation analysis demonstrated good convergent validity. Significant negative correlations were found with body weight, body mass index (BMI), waist circumference, and body fat (all <i>p</i> &lt;0.001). Significant positive correlations were found with vigorous intensity activity (<i>p</i> &lt;0.001), moderate intensity activity (<i>p</i> = .004), walking (<i>p</i> = .005), and quality of life (<i>p</i> &lt;0.001). Good known-groups validity was demonstrated by significant differences between habitual physical activity levels for body weight, waist circumference, body fat, vigorous intensity activity (all <i>p</i> &lt;0.001), moderate intensity activity (<i>p</i> = .038), walking (<i>p</i> = .049), and quality of life (<i>p</i> &lt;0.001). Regression models confirmed known-groups validity.</p> Conclusions <p>A single question with categorical descriptors is valid for brief clinical assessment of habitual physical activity in women across a wide age range.</p>