342 With a 2-day episode length, 33% of all hypomania remained outside of an episode. There was no signiﬁcant difference in the percent of hypomanic days outside of an episode between patients with bipolar I and II dis- orders. There were no signiﬁcant differences in the demographic characteristics of patients who met the 4-day minimum as compared with those who only experienced episodes of hypomania using a shortened length criterion. Decreasing the minimum length criterion for an episode of hypomania will cause a large increase in the number of patients who experience an episode and in the aggregate number of episodes, but will not distinguish subgroups within a sample who meet the DSM-IV criteria for bipolar disorder. Frequency may be an important dimensional aspect of brief hypomania. Clinicians should regularly probe for brief hypomania. Keywords hypomania Bipolar disorder, hypomania DSM-IV criteria Brief Introduction Many patients with bipolar disorder experience hypoma- nia for periods less than the minimum episode length criterion of 4 days in the DSM-IV [1–4]. There is a need to increase focus on this brief hypomania since even subsyndromal symptoms may impair functioning and diminish the quality of life [5–7]. Additionally, some researchers feel that a shorter length criterion would better reﬂect the entire spectrum of bipolar disorder, and improve detection of bipolarity in patients with depressive episodes [2, 3, 8–10]. The criteria used to deﬁne an epi- sode of hypomania are under revision as part of the ongoing DSM-V review process [11]. Within this context, the primary objective of this study was to characterize brief hypomania lasting less than 4 days that is experi- enced by patients who meet the DSM-IV criteria for bipolar disorder. The secondary objective was to deter- mine if the patients who experienced an episode of hypomania only when using a shortened criterion were a distinct subgroup from those who met the 4-day criterion during the study period. We have previously investigated the impact of changing the length criterion for an episode of hypomania from 4 days to 2 using daily self-reported mood ratings from 203 patients who were diagnosed with bipolar disorder using the DSM-IV criteria [4]. As the minimum length criterion for an episode of hypomania decreased, a large increase was found in both the number of patients with episodes and the number of episodes. This study repeated the prior analysis using a larger sample and including a length cri- terion of a single day. 123 Eur Arch Psychiatry Clin Neurosci (2011) 261:341–347 Methods The inclusion criteria were a diagnosis of bipolar disorder by DSM-IV criteria, age 18 years or older, currently receiving pharmacological treatment and a willingness to record mood daily for 5 months using computer software in their native language. All participants volunteered, pro- vided informed written consent, and received treatment as usual throughout the study. The naturalistic approach with minimal exclusion criteria was selected to better reﬂect routine clinical practice and patient heterogeneity. The diagnosis of bipolar disorder was made by the prescribing psychiatrist in a clinical interview. Additional details about the ChronoRecord study were published previously [12]. Patients from our prior study [4] were included in this analysis, although some provided additional data. Daily mood ratings Patients provided a daily mood rating using the previously validated ChronoRecord software that was installed on a home computer [13, 14]. A 100-unit visual analogue scale was used to rate mood between the most extreme mania and depression the patient ever experienced. Based upon the validation studies comparing the self-ratings with cli- nician ratings on the Hamilton Depression Rating Scale (HAMD) and the Young Mania Rating Scale (YMRS) [13, 14], a mood entry less than 40 was considered depression, 40–60 euthymia, and greater than 60 hypomania/mania. A rating of 61–80 was considered hypomania and 81–100 was considered mania. The self-ratings of mania reﬂect activation levels for either euphoric or dysphoric mood [13]. A detailed account of data collection using Chrono- Record is provided elsewhere [12–14]. Statistics The demographic characteristics, mood ratings, and psy- chotropic medications taken by the patients were obtained. To be considered using a medication, a patient had to take any dose of the drug for at least 50% of the days. Episodes of hypomania and depression were determined using a published algorithm to calculate episodes from daily self- reported mood data based on the DSM-IV criteria [15]. Episodes of hypomania were also calculated while varying the minimum duration between 3, 2, and 1 days. The demographic characteristics and medications taken were compared between patients with bipolar I and bipolar II disorder, using the Pearson 2-sided V2 test for distributions and the independent sample 2-sided t test for mean values. For the subgroup analysis, the demographic characteristics