A bstract T his study investigated the relationship between locus of control and self-reported adherence in 31 adults with cystic ® brosis. Participants com pleted the M ultidimensional H ealth Locus of Control Scale, Form C (M H LC -C;W allston et al., 1994) and an adherence questionnaire. The two`powerful others' sub-scales of the M HLC-C ,`doctors' and`others' , were signi® cantly correlated with overall self-reported adherence. M ultiple regression indicated that the`doc tors' sub-scale explained 35% of the variance in overall adherence. It is concluded that an external locus of control for powerful others, especially doctors, m ay prom ote greater adherence.C ystic ® brosis (CF) is a serious genetic disorder which requires com plex daily life-long com m itm ent to m edical regim ens which are necessary to both m inimize and delay the effects of th e disease process (Harris & Super, 1995). The m ain patho physiology of CF is observed in the lungs and digestive system and treatm ent includes replacem ent of pancreatic enzym es and nutritional supplem ent, antibiotics and chest physiotherapy (Glasscoe, 1997). The advent of the C F centres, early diagnosis and improved treatm ents have led to the prediction that 50% of patients with CF will reach adulthood (Glasscoe, 1997).H ow ever, whatever th e advan ces in treatm ent, an importan t factor is how m any treatm ents patients adh ere to and what factors in¯uence such adherence. Non-adherence to m edical treatm ent is a widespread problem (see M yers & M idence, 1998), with adherence to long-te rm regim ens being estim ated at around 50% (Sackett & Snow , 1979). Research on C F has indicated that adherence is a problem in both children and adults (Abbo tt et al., 1994; for a recent review see Bryon, 1998). A search for factors which are associated with poor adherence in CF has been inconclusive. H ow ever, in a recent study of adults with CF, Abbo tt et al. (1996) m easured health locus of control (HLC ) with the generic M ultidimensional H ealth Locus of C ontrol Scale (M H LC ; W allston et al., 1978), the m ost com m only used m easure of H LC. The M HLC recognizes different control beliefs which are m easured by three separate scales: an internal scale, and