Increased variability is a characteristic clinical and physiologic feature of functional (psychogenic) tremor. In this study, we use computerized spiral analysis to show that the variability of a motor task is a quantifiable characteristic of functional tremor. We compare functional tremor patients to phenomenologically similar dystonic tremor patients and to normal controls. We used the spiral severity score, a measure that does not incorporate spiral tightness, as a marker of spiral drawing performance, and inter-spiral tightness variability (based on the 25-75%(ile) range in tightness across ten spirals) to evaluate the effects of functional tremor on drawing spirals. The spirals of 74 participants: 22 functional tremor, 21 dystonic tremor, and 31 normal controls were analyzed. Spiral severity was higher in both tremor groups compared to controls, but did not differentiate them. Inter-spiral variability, however, was higher in the functional tremor group compared to both other groups. Thus, spiral analysis captures variability of a motor task and may be used as an objective test for functional tremor. The effect of functional tremor in other motor tasks should be investigated.