This study investigates whether illness representations predict changes in asthma control or vice versa. Illness perceptions and asthma control were assessed in N = 113 asthma patients at the begin/end of an inpatient rehabilitation. Bivariate cross-lagged panel analyses showed that the illness representation dimensions Personal control, Consequences, Coherence, and Emotional representation predicted the change in asthma control, but asthma control did not predict illness representations. In multiple regression analyses with covariates, Personal control still predicted the change in asthma control. Illness beliefs assessed at the beginning of an intervention might help to identify patients with lower probability of treatment success.