The aim of our study was to assess the frequency of depression in group of patients with Parkinson's disease (PD) who fulfilled the diagnostic criteria of PD, had normal CT scans and responded well to L-dopa treatment. The sample consisted of 73 consecutive patients (34 women and 39 men), mean age 65.7 (41-81) years, mean duration of disease 6.7 years. Besides neurological examination, in all the patients the degree of motor impairment was evaluated using the UPDRS, H-Y, and SE scales. Moreover, a sociodemographic questionnaire, psychological tests (MADRS, MMSE), and a quality of life scale (PDQ-39) were used. Depression (MADRS scores > 19) was found in 25 (34.2%) of the patients, with major depression (scores > 28) diagnosed in 7 patients (9.5%) and moderate depression (scores between 20 and 28)--in 18 cases (24.6%). In comparison to non-depressed patients, those with depression were older by 0.9 years on the average, their onset of the disease occurred later by 1.7 years, and their mean duration of the disease was longer by 2.6 years. These differences were not statistically significant. Dementia (MMSE scores < or = 23) did not differentiate between the two groups: it was found in 27 depressed patients (37.4%) and in 26 (35.6%) of those without depression. Patients in the depressed group suffered statistically more often from sleep disorders (19 vs. 14; p < 0.001). In this group motor impairment was significantly more marked, as measured by the UPDRS (32.2 vs. 46.8; p < 0.001) and H-Y (2.54 vs 2.98; p < 0.007), and their quality of life as measured by PDQ-39 questionnaire was significantly lower (36.4 vs. 82.24; p < 0.00002). Our data indicate the presence of depression in 34.2% of the sample, i.e. a somewhat lower prevalence rate than that reported in other studies. This may be due to the fact that only outpatient population was analysed, and outpatients are seldom categorized as degree 4 and 5 on the H-Y scale. Depression on PD patients was correlated with their more severe motor disability and considerably lower quality of life. This may suggest a relationship with progression of the disease and more pronounced changes in cerebral neurotransmitters (i.e. endogenous origin), or PD patient's response to their limited mobility and isolation in later stages of the disease (i.e. reactive origin). However, the two factors--endogenous and reactive--may be overlapping, since a majority of PD patients suffer from mild to moderate depression.