BACKGROUND: Contemporary models of pain suggest the beliefs an individual holds regarding the cause of pain may influence pain intensity. Direct evidence supporting this idea is lacking in clinical populations. OBJECTIVE: Investigate if the content of diagnostic information about the cause of pain influences pain and function in people with Achilles tendinopathy. METHODS: Fifty recreational runners with Achilles tendinopathy participated in this randomised clinical experiment. Participants in the experimental group received an explanation of tendon pain that highlighted changes in muscle function as the primary problem, without reference to tendon pathology. Participants in the control group received an explanation of tendon pain that prioritised tendon pathology. The primary outcome measure was maximal pain intensity during a standardised hopping task measured on a 0-100 visual analogue scale. Secondary outcomes were lower limb stiffness while hopping and time in seconds for pain to ease upon completion of the hopping task. RESULTS: Pain intensity was lower in the experimental group post intervention (mean difference = 12.3 mm; 95 % CI: 3.2, 21.5). Lower limb stiffness was higher in the experimental group (mean difference = -1546 Nm; 95 % CI: -3296, 204) and time-to-ease was near identical (mean difference = 0 s; 95 % CI: -11, 11). CONCLUSION: The content of diagnostic information influenced pain intensity with loading in people with Achilles tendinopathy. The effect on lower limb stiffness was uncertain and there appears to be no effect on time-to-ease.