The literature regarding parkinsonism of early-onset indicates that it encompasses several diseases differing in their clinical and pathological features. Since 1968 we have reported cases of early-onset parkinsonism with diurnal fluctuation of symptoms (abbrev. as EPDF). Attention has been focused on its similarities to or differences from Parkinson's disease (PD), juvenile parkinsonism of Yokochi, and hereditary progressive dystonia (HPD) of Segawa et al. In this paper we report the clinical and pathological characteristics of EPDF to facilitate its nosological identification. We examined the pathological features of EPDF in two cases. Case 1 was a 52-year-old female with consanguineously related parents and two other affected sisters. The disease began at the age of 20, and she had marked diurnal fluctuation of symptoms. With the assistance of L-dopa and bromocriptine, she was able to enjoy relatively satisfactory daily life activity until the age of 50. In the last two years she was bed-ridden with advanced parkinsonism. Case 2 was a 56-year-old man without any contributing family history. His disease started at age 26 and his symptoms showed slight fluctuation during the earlier stages of the disease. Treatment with L-dopa and bromocriptine was associated with marked up-and-down phenomenon and choreatic dyskinesia. Pathological study in the two cases revealed marked cell loss in the substantia nigra zona compacta, especially in the area A9, while the neuronal cell population of the ventral tegmental area (A10), locus caeruleus, superior raphae nucleus, and substantia innominata was relatively well preserved. There were no Lewy bodies.(ABSTRACT TRUNCATED AT 250 WORDS)