Illness representations were assessed in 63 adults with rheumatoid arthritis (RA) and 66 with multiple sclerosis (MS). The relationship of illness representations to concurrent and later mood was explored. MS patients' beliefs in symptom variability were associated with higher depressed mood 4 months later, over and above initial levels of depression. RA patients who saw RA as curable or who saw themselves as responsible for the illness reported significant increases in depression over time. Belief in the serious consequences of RA interacted with later illness severity to predict change in depression. When belief in the serious consequences of RA was high, less severe illness status was associated with less depression and more severe illness status was associated with more depression. When RA was initially viewed as only moderately serious, less severe illness was associated with somewhat higher levels of depression.