A meta-analysis was conducted on 25 longitudinal studies involving 901 initially non-demented Parkinson's disease (PD) patients to examine the magnitude of decline across multiple cognitive domains associated with disease progression. Pooled effect sizes reflecting the standardized difference between baseline and follow-up neuropsychological performance were calculated for 8 cognitive domains using a random-effects model. Relatively small effect sizes were found across all cognitive domains (d = .00 - .40). During a mean follow-up interval of 29 months, significant declines were detected in global cognitive ability (d = .40), visuoconstructive skills (d = .32), and memory (d = .29). Age showed a significant relation with decline in global cognitive ability and memory. Lower educational level was associated with greater decline in all cognitive domains. Studies with longer follow-up intervals yielded larger effect sizes for global cognitive ability. In non-demented PD patients, changes in cognitive functions over time appear to be modest. Educational level, age, and length of the follow-up interval are likely to affect the magnitude of decline in several domains. Methodological flaws, such as selection bias and uncontrolled practice effects, may have caused underestimation of the true extent of decline.