Brief Report Admission rates of bipolar depressed patients increase during spring/summer and correlate with maximal environmental temperature Seasonality in the evolvement of depressive symp- toms [which are not part of seasonal affective disorder (SAD) (1)] has been reported for both unipolar and bipolar patients but findings are inconsistent (2–8). Moreover, as far as we know, there is no data in the literature on the potential impact of specific climatic parameters on admis- sion rates to psychiatric hospitals of non-SAD patients with major depressive disorder [either unipolar (UPD) or bipolar (BPD)]. Hence, our main objective was to identify a possible correlation between various climatic fac- tors and the admission rates to psychiatric hospi- tals of patients with ICD-9 major depressive disorder, either unipolar or as part of bipolar affective disorder. Methods Subjects The study group consisted of all patients diag- nosed as having UPD or BPD (ICD-9 code: 296.2/296.3 or 296.5, respectively) who were Shapira A, Shiloh R, Potchter O, Hermesh H, Popper M, Weizman A. Admission rates of bipolar depressed patients increase during spring/ summer and correlate with maximal environmental temperature. Bipolar Disord 2004: 6: 90–93. ª Blackwell Munksgaard, 2004 Objective: We intended to identify a relationship, if exists, between various climatic factors and the admission rates of bipolar affective disorder depressed patients (BPD) or major depressive disorder patients (unipolar) (UPD) to psychiatric hospitals, as well as potential seasonal variability in hospitalization rates of this population. Methods: Data on admissions of ICD-9 BPD and UPD patients to Tel Aviv’s seven public psychiatric hospitals during 11 consecutive years were collected along with concomitant meteorological information Results: Admissions of 4117 patients with BPD and 1036 with UPD who fulfilled our specific inclusion criteria were recorded. Bipolar depressed, but not UPD, patients exhibited significant seasonal variation (higher spring and summer versus winter mean monthly admission rates), and the admission rates of patients with BPD, but not UPD, correlated significantly with mean maximal monthly environmental temperature Conclusions: Increased environmental temperature may be a risk factor for evolvement of major depressive episode in patients with bipolar disorder with psychiatric co-morbidity, at least in cases that necessitate hospitalization and at the examined geographic/climatic region of Israel. Further large-scale studies with bipolar depressed patients with and without co-morbid disorders are needed to substantiate our findings and to determine the role of seasonal and climatic influence on this population, as well as its relationship to the pathophysiology of bipolar disorder. Avraham Shapira a , Roni Shiloh b , Oded Potchter c , Haggai Hermesh b , Miriam Popper d and Abraham Weizman b a Psychiatric Division, Rambam Medical Center, Bruce Rappaport Faculty of Medicine, Technion, Haifa, b Geha Psychiatric Hospital, Felsenstein Medical Research Center, Rabin Medical Center, Beilinson Campus, Petah-Tikva, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, c Department of Geography and Human Environment, Tel Aviv University, Tel Aviv, d Department of Information and Evaluation, Mental Health Services, Ministry of Health, Israel Key words: bipolar affective disorder – climate – depression – environmental temperature – major depressive disorder (unipolar) – seasonality Received 30 December 2002, revised and accepted for publication 4 August 2003 Corresponding author: Roni Shiloh MD, Geha Psychiatric Hospital, Felsenstein Medical Research Center, Rabin Medical Center, Beilinson Campus, PO Box 102, Petah-Tikva 49100, Israel. Fax: +972-3-9241041; e-mail: shiloh_r@netvision.net.il There are no commercial associations related to any of the authors that might pose a conflict of interest in connection with the manu- script. Bipolar Disorders 2004: 6: 90–93 Copyright ª Blackwell Munksgaard 2004 BIPOLAR DISORDERS 90