Electronic caps, pill caps that record the date and time of pill bottle opening provide an objective measure of adherence to prescribed medication. A promising intervention to improve adherence, cue-dose training, involves reviewing patients' pill cap-generated reports concerning their medication-taking and offering individualized recommendations for remembering to take medications at specific times of day. In this preliminary study, 79 patients prescribed the antihyperglycemic medication metformin had adherence assessed during a 4-week baseline period. Adherence, defined as proportion of prescribed doses taken within a predetermined 4-h window, was measured using electronic MEMS caps. Those who had less than 80% baseline adherence (n = 33) were randomly assigned to either receive 4 months of cue-dose training (n = 16) or to a control group (n = 17). Cue-dose training was associated with significantly better adherence to metformin (mean improvement of 15%). The effects of cue-dose training on adherence to other antihyperglycemic medication did not reach statistical significance. Glycosylated hemoglobin (a measure of blood sugar control) did not differ between groups. Data from nine patients who reviewed pill cap-generated data with their primary care providers suggested that both patients and providers found the discussion moderately helpful and not at all uncomfortable.