Background: In English- and German-speaking countries, ataxic speech is often described as showing scanning based on acoustic impressions. Although the term "scanning" is generally considered to represent abnormal speech features including prosodic excess or insufficiency, any precise acoustic analysis of ataxic speech has not been performed in Japanese-speaking patients. This raises the question of what is the most dominant acoustic characteristic of ataxic speech in Japanese subjects, particularly related to the perceptual impression of "scanning". Aims: The study was designed to investigate the nature of speech characteristics of Japanese ataxic subjects, particularly "scanning", by means of acoustic analysis. Methods & Procedures: The study comprised 20 Japanese cases with spinocerebellar degeneration diagnosed to have a perceptual impression of scanning by neurologists (ataxic group) and 20 age-matched normal healthy subjects (control group). Recordings of speech samples of Japanese test sentences were obtained from each subject. The recorded and digitized acoustic samples were analysed using "Acoustic Core-8" (Arcadia Inc.). Outcomes & Results: Sentence duration was significantly longer in the ataxic group as compared with the control group, indicating that the speaking rate was slower in the ataxic subjects. Segment duration remained consistent in both vowels and consonants in the control group as compared with the ataxic group. In particular, the duration of vowel segments, i.e. the nucleus of Japanese mora, was significantly invariable in the control group regardless of differences between subjects as well as in segments compared with the ataxic group. In addition, the duration of phonemically long Japanese vowels was significantly shorter in the ataxic group. Conclusions & Implications: The results indicate that the perceptual impression of "scanning" in Japanese ataxic cases derives mainly from the breakdown of isochrony in terms of difficulty in keeping the length of vowel segments of Japanese invariable during speech production. In addition, the tendency toward irregular shortening of the length of phonemically long Japanese vowels is thought to reinforce the impression of "scanning" in ataxic speech in Japanese cases. (Contains 2 tables and 6 figures.)