The literature concerning the clinical and pharmacologic studies of systemically administered timolol in 2233 patients was reviewed. Side effects reported were correlated with dosage levels and estimated plasma levels. Most adverse effects occurred with levels much higher than would be expected after the topical use of timolol to control glaucoma. The table of potential complications and adverse effects seen with the use of systemic timolol is meant to focus attention on expected and unexpected side effects. All could occur and should be looked for with the topical use of this drug. The goal of this paper is to define better what physicians should look for in evaluating the clinical safety of this drug and what can be realistically expected to be seen as the use of this drug becomes more widespread.