Background: Chronically ill patients such as people with type 2 diabetes develop perceptions of their illness, which will influence their coping behaviour. Perceptions are formed once a health threat has been recognised. Many people with type 2 diabetes suffer from multimorbidity, for example the combination with cardiovascular disease. Perceptions of one illness may influence perceptions of the other condition. The aim of the current study was to evaluate the effect of an intervention in type 2 diabetes patients with a first acute coronary event on change in illness perceptions and whether this mediates the intervention effect on health status. The current study is a secondary data analysis of a RCT. Methods: Two hundred one participants were randomised (1:1 ratio) to the intervention (n = 101, three home visits) or control group (n = 100). Outcome variables were diabetes and acute coronary event perceptions, assessed with the two separate Brief Illness Perceptions Questionnaires (BIPQs); and health status (Euroqol Visual Analog Scale (EQ-VAS)). The intervention effect was analysed using ANCOVA. Linear regression analyses were used to assess whether illness perceptions mediated the intervention effect on health status. Results: A positive intervention effect was found on the BIPQ diabetes items coherence and treatment control (F = 8.19, p = 0.005; F = 14.01, p < 0.001). No intervention effect was found on the other BIPQ diabetes items consequence, personal control, identity, illness concern and emotional representation. Regarding the acute coronary event, a positive intervention effect on treatment control was found (F = 7.81, p = 0.006). No intervention effect was found on the other items of the acute coronary event BIPQ. Better diabetes coherence was associated with improved health status, whereas perceiving more treatment control was not. The mediating effect of the diabetes perception 'coherence' on health status was not significant. Conclusion: Targeting illness perceptions of people with diabetes after an acute coronary event has no effect on most domains, but can improve the perceived understanding of their diabetes. Discussing perceptions prevents people with type 2 diabetes who recently experienced an acute coronary event from the perception that they will lose control of both their diabetes and the acute coronary event. Illness perceptions of diabetes patients should therefore be discussed in the dynamic period after an acute coronary event. Trial registration: Nederlands trial register; NTR3076, Registered September 20 2011.