With the aim of comparing the previously found high prevalence of idiopathic Parkinson's disease (PD) in the Faroe Islands with the prevalence of PD in an area of Denmark, we used the same case-finding methods for case ascertainment and the same strict criteria to diagnose PD on the island of Als. During the last year before the prevalence date (1 January 1998), we found in various registries from pharmacies, hospital, private neurologist and general practitioners 121 patients with suspected Parkinsonism out of 56,839 inhabitants on the island of Als. After exclusion of those who had other diseases, a total of 79 patients were left for further examinations. Among these we found 58 with PD. The overall prevalence of PD was estimated to be 102.0 and the age-adjusted prevalence to be 98.3 per 100,000 persons compared with 187.6 and 209.0 in the Faroe Islands. Compared with the previous results from the Faroe Islands (prevalence date 1 July 1995) we found an even lower mean age at onset of PD symptoms and at onset of treatment, a lower proportion of definite PD and a lower average dose of levodopa. We therefore conclude that the two-fold higher prevalence in the Faroe Islands than on the island of Als was not due to an early diagnosis and a higher ascertainment of cases with mild PD, which was suggested as being one possible explanation for our previous finding of a high prevalence of PD in the Faroe Islands.