Chronic obstructive pulmonary disease (COPD), diabetes and asthma are chronic illnesses that affect a substantial number of people. The continued high cost of clinic- and hospital-based care provision in these areas could be reduced by patients self-monitoring their condition more effectively. Such a move requires an understanding of how to predict self-monitoring compliance. Ajzen's theory of planned behaviour (TPB) makes it possible to predict those clients who will comply with medical guidelines, prescription drug intake and self-monitoring behaviours (peak flow or blood sugar levels). Ninety-seven clients attending a medical centre located in a large urbanised area of Northern Ireland completed TPB questionnaires. Significant amounts of variance explained by the TPB model indicated its usefulness as a predictor of self-monitoring behaviour intentions in the sample. The results also highlighted the importance of subjective norm and perceived behavioural control within the TPB in predicting intentions. The utility of the TPB in this study also provides evidence for health promotion professionals that costly clinic/hospital treatment provision can be reduced, whilst also being satisfied with ongoing client self-monitoring of their condition.