The main objectives of this study were firstly, to compare candidate measures of adherence (cyclosporin levels, self-report, clinician and interviewer ratings) used in all subjects with the 'gold standard' of electronic monitoring used in a random sample of 60 subjects to find the most valid and feasible method for use in clinical practice, and estimate the prevalence of non-adherence; secondly, to investigate the major variables associated with non-adherence with the aim of identifying potentially modifiable factors. One hundred and seventy two subjects were invited to take part. Nineteen refused to take part and complete data were available for 145 subjects. The sample was representative of the eligible population. Other measures of adherence performed poorly when tested against electronic monitoring. Therefore main analyses were confined to the 58 subjects with data available from electronic monitoring of adherence to prednisolone. Two patterns of non-adherence were identified: missing medication and erratic timing of doses. Seven (12%) subjects missed at least 20% days prednisolone and 26 (45%) took their medication outside a 12 hour period 32% of the time. Multivariate analyses showed that the factors most strongly associated with non-adherence were having a transplant from a live donor, having less belief in the need for prednisolone specifically or for immunosuppressants as a group, being prescribed prednisolone on alternate days and functional limitations due to emotional factors. Depression occurred in 30% subjects but was not significantly associated with non-adherence. The results show that a significant proportion of transplant recipients are non-adherent to immunosuppressants and that beliefs about medication are a promising target for an intervention designed to improve adherence. Clinicians need to be aware that subjects with transplants from live donors may be at greater risk of non-adherence and that patients may hold beliefs about immunosuppressants that differ from medical opinion and that impair adherence in manner that is logical to the patient.