Amyotrophic lateral sclerosis (ALS) is a terminal neurological disease associated with progressive paralysis, loss of communicative ability and functional decline. Expressive disclosure may help people with ALS, particularly those who are emotionally or socially inhibited, meet psychological challenges associated with the disease. People with ALS (N = 48) were randomised to expressive disclosure about their disease or no disclosure. Psychological well-being (affect, depression and quality of life) was assessed pre-intervention and also three and six months later. Results of multi-level models indicated that the group that disclosed thoughts and feelings about ALS had higher well-being than the control group at three months post-intervention, but not six months. Ambivalence over emotional expression (AEE) moderated three-month post-intervention well-being. Those low in AEE had higher well-being than those high in AEE regardless of condition. Those high in AEE, who disclosed, had increased well-being from pre-intervention, whereas controls had decreased well-being from pre-intervention. Expressive disclosure may be helpful for people with ALS, but only those who have difficulty expressing emotions. In addition, the intervention had only temporary effects; the dynamic challenges of ALS progression may mean that the effect of processing thoughts and feelings about the disease in one stage may not generalise to later stages.