The aim of this cross-sectional study was to quantify patients' personal beliefs about the necessity of their prescribed medication and their concerns about taking it and to assess relations between beliefs and reported adherence among 324 patients from four chronic illness groups (asthma, renal, cardiac, and oncology). The findings revealed considerable variation in reported adherence and beliefs about medicines within and between illness groups. Most patients (89%) believed that their prescribed medication was necessary for maintaining health. However, over a third had strong concerns about their medication based on beliefs about the dangers of dependence or long-term effects. Beliefs about medicines were related to reported adherence: higher necessity scores correlated with higher reported adherence (r=0.21, n=324, p<0.01) and higher concerns correlated with lower reported adherence (r=0.33, n=324, p<0.01). For 17% of the total sample, concerns scores exceeded necessity scores and these patients reported significantly lower adherence rates (t=-4.28, p<0.001). Stepwise multiple linear regression analysis showed that higher reported adherence rates were associated with higher necessity-concerns difference scores (beta=0.35, p<0.001), a diagnosis of asthma (beta= -0.31, p<0.001), a diagnosis of heart disease (beta=0.19, p<0.001), and age (beta=0.22, p<0.001). Gender, educational experience, or the number of prescribed medicines did not predict reported adherence. Medication beliefs were more powerful predictors of reported adherence than the clinical and sociodemographic factors, accounting for 19% of the explained variance in adherence. These data were consistent with the hypothesis that many patients engage in an implicit cost-benefit analysis in which beliefs about the necessity of their medication are weighed against concerns about the potential adverse effects of taking it and that these beliefs are related to medication adherence.