In the 19th century, deaths from acute exposure to hydrogen sulfide (H2S) portended permanent brain injury from nonlethal doses. The neurobehavioral effects of H2S exposures lasting from moments to years were compared in 16 subjects, 2 years to 22 years afterward. Neurophysiologic and psychologic tests were used to appraise mood status and frequencies of 35 symptoms. Functions and frequencies, described as percent predicted adjusted for age, sex, educational achievement, and other factors, were compared with those in an unexposed population. Frequencies were elevated for 31 of 33 symptoms. Balance was impaired (246% predicted with eyes closed, 159% predicted with eyes open), and simple and choice reaction times were prolonged (151% and 130% predicted, respectively). Visual fields performance was decreased to 72% predicted (right) and 55% predicted (left), color discrimination was abnormal, and hearing was decreased. Psychologic domains showed cognitive disability, reduced perceptual motor speed, impaired verbal recall and remote memory, and abnormal mood status. Exposure to H2S must be avoided.