Background. Data on the efficacy and safety of methylphenidate in adults with attention deficit/hyperactivity disorder (ADHD) are lacking in Europe. This study was undertaken to report on the efficacy and safety of methylphenidate in an adult out-patient population with ADHD, and to compare results with US data. Method. A double-blind randomized cross-over trial comparing methylphenidate and placebo in 45 adults with ADHD with childhood onset was performed in a dose-titration design. Methylphenidate was titrated from 0·5 mg/kg per day in week 1 up to 1·0 mg/kg per day in week 3. Results. Response rates using methylphenidate varied between 38 and 51%, and using placebo between 7 and 18% (p<0·05), depending on outcome measure used. Although the overall percentage of subjects having any side effect on both methylphenidate and placebo was rather high, side effects on methylphenidate over and above those on placebo were few and mild. Conclusions. Methylphenidate proves to be an effective and well tolerated treatment for symptoms of ADHD in adults in the short term. Future research should study the long-term response and clarify the impact of gender, co-morbidity, socio-economic status and IQ on response rates in adults with ADHD. © 2004 Cambridge University Press.