Written information to promote patient education is being increasingly recognized as an integral part of quality health care. The main objective of this study was to evaluate the effect of distributing a patient information leaflet (PIL) on knowledge acquisition and recall. Two different PILs were designed for co-trimoxazole tablets: a simple, shorter PIL that incorporated pictograms and text and a text-only PIL that was longer and more complex. Human immunodeficiency virus-positive participants on chronic co-trimoxazole therapy were enrolled from five local primary health care clinics in Grahamstown, South Africa, and were randomly allocated to a Control Group (no PIL), Group A (text-only PIL) or Group B (simple PIL with pictograms). At the preliminary interview, demographic data were collected and the tablets dispensed according to normal clinic protocol. In a follow-up interview approximately 14 days later, participant medicines knowledge was investigated by asking a series of questions. The mean percentage for medicines knowledge was significantly higher in the group that received the simple PIL incorporating pictograms (76.3%), compared with both the Control Group (43.3%) and the group who received the longer, text-only PIL (50.9%). This study reinforces the value of providing patients with an appropriately designed PIL to inform appropriate medicine-taking behaviour.