context: A clinical trial, also called a research study, is a research program that tests the effectiveness and safety of various interventions for people with a certain condition. Clinical trials are meant to find new and improved methods of evaluating or treating a condition. They also can test new ways to prevent diseases. Such trials can involve risks and there is no guarantee regarding a trial's outcome. Clinical trials are conducted in phases and may span several weeks to several years. Clinical trials are generally divided into four phases: Phase I clinical trials involve giving a new treatment to a small number of participants. The researchers determine the best way to give the new treatment and how much of it can be given safely. Some Phase I trials have a limited number of participants who would not be helped by other known treatments. Other Phase I trials are performed in healthy volunteers to determine the safety of a particular treatment. Phase II clinical trials focus on learning whether the new treatment has an effect on a specific condition. Additional information regarding the side effects of the treatment is also obtained. A small number of people are included because of the risks and unknowns involved. Phase III clinical trials compare the new treatment with a placebo or a standard treatment. In this phase, researchers determine which study group has fewer side effects and is demonstrating the most improvement. Phase IV clinical trials, also called post-marketing studies, are conducted after a treatment has been approved. The purpose of these trials is to provide an opportunity to learn more details about the treatment and to address questions that may have come up during other phases of trials. This phase involves many more people and may identify side effects that were not previously reported during other phases. Clinical trial participants are assigned at random (a process similar to flipping a coin) to either the new treatment (treatment group) or the current standard treatment (control group). Randomization helps to avoid bias (having the study's results affected by human choices or other factors not related to the treatments being tested). When no standard treatment exists for a condition, some studies compare a new treatment with a placebo (a look-alike pill/infusion that contains no active drug). However, a person is told this is a possibility before deciding whether or not to take part in a study. In a clinical trial, patients receive treatment and researchers observe how the treatment affects patients. The patient's progress is closely monitored during the trial. Once the treatment portion of the trial has been completed, researchers may continue to follow patients in order to gather more information about the effects of a treatment. While clinical trials have risks for the people who take part, each study also takes steps to protect patients. Only you can decide whether taking part in a clinical trial is worthwhile. The possible benefits and risks should be considered carefully. What is the purpose of the study? What has previous research of this treatment shown? What is likely to happen in my case with or without the treatment? Are there standard treatments for this condition? How does this study compare with standard treatment options? Many trials for the diagnosis and treatment of ADHD in children have been conducted. These have included trials measuring drug treatment, behavioral treatment, and/or combined treatments for children. Some clinical trials involving children have tested the potential roles that lead exposure, exposure to other neurotoxins, and psychosocial situations (involving aspects of social and psychological behavior) might play in causing ADHD. For the most current listing of clinical trials in the field of ADHD, please consult the website www.clinicaltrials.gov and conduct a search under the term ADHD.