In a 6-month longitudinal randomized field experiment, we examined how using height-adjustable sit-stand desks could have beneficial, adverse, and spiraling effects on people's musculoskeletal and psychovegetative complaints, and on positive (vitality and vigilance) and negative psychological symptoms, namely, stressor uncontrollability (i.e., perceived uncontrollability of workload), psychological tension, and mental tiredness. A total of 127 employees in various, mostly sedentary, occupations were randomly assigned to either the intervention or the control group. Variables were assessed monthly for 6 months on a self-reported basis. Bayesian structural equation modeling showed that the intervention produced large inhibiting between-subjects effects for musculoskeletal problems in the neck, back, and shoulders (β ranged between -.26 and -.21). Within-subject analyses revealed that the intervention produced large inhibiting effect sizes for intensity (g = 3.06) and prevalence of musculoskeletal (g = 1.19) and psychovegetative complaints (g between 0.76 and 1.57). For negative psychological symptoms (i.e., psychological tension and mental tiredness), participants in the intervention group showed a steeper decrease than participants in the control group (g between 2.34 and 3.74). For positive indicators (i.e., vitality and vigilance), the intervention produced large promoting effects for participants in the intervention group compared with participants in the control group (g between 0.70 and 1.65). There was no change in stressor uncontrollability between the two groups. Finally, findings suggest that sit-stand desks can be effective in improving occupational health by weakening a downward-spiraling effect. (PsycINFO Database Record (c) 2020 APA, all rights reserved).