Two women aged 50 and 64 years, respectively, and one man aged 43 years, were treated with cyclic etidronic acid for osteoporosis. After some months/years they developed mood, concentration and memory problems. The complaints diminished within several weeks after withdrawal of the drugs, and reappeared after rechallenge. Two of these patients had similar complaints during the use of another bisphosphonate (pamidronic acid and alendronic acid, respectively). Bisphosphonates are used increasingly frequently for the treatment of osteoporosis. Generally, these drugs are well tolerated. The most frequently reported adverse events are gastrointestinal complaints and oesophageal ulcers. Psychiatric complaints also appear to be a rare side effect.