In 19 patients with severe Parkinson's disease, we replicated the surgical procedures developed by Madrazo et al. for transplantation of the adrenal medulla to the striatum, and followed them for six months after operation. We monitored their motor function with the use of standardized scales and determined the amount and quality of "on" and "off" time (the hours of the waking day when the antiparkinsonian medications were effective and ineffective, respectively). We found significant improvement in focal areas of motor function. The mean percentage of on time during the day increased from 47.6 percent to 75.0 percent (P = 0.012); the mean percentage of on time without chorea increased from 26.6 percent to 59.2 percent (P = 0.006); the mean severity of off time decreased as assessed by both the Activities of Daily Living subscale of the Unified Parkinson's Disease Scale (P = 0.002) and the Schwab and England scale (P = 0.037). In contrast to the finding of Madrazo et al., however, the dosages of antiparkinsonian medications could not be decreased and postoperative morbidity was substantial. Despite cautious optimism, we conclude that the widespread use of this procedure outside of research centers is premature, since the improvement we found was slighter than in the previous cases.