This study was done in an attempt to clarify the significance of balneal reaction, which was often observed in the course of balneotherapy. Balneal reaction has been known for long time as “Yuatari” but no systematic study has been carried out. We have been studying balneal reaction systematically and in detail with respect to frequency, symptoms, course of development and various kinds of vital reactions at several hot springs in Tohoku Region.<br>This paper is a summary of these studies.<br>According to the statistic survey, the frequency of balneal reaction was estimated to be 15-41%. Incidence was high in acid and vitriol springs. The statistics made from the accurate survey of 1086 bathers at Narugo Spa, however, revealed that the frequency of balneal reaction was 80.6%. This figure was greatly different from the 40.9%, calculated from the subjective realization of balneal reaction in same group. Of these bathers, 79.2% showed generalized symptoms, 9.5% showed localized symptoms and 10.3% showed symptoms with pronounced feeling of sickness.<br>Frequency greatly differed with the chemical properties of hot springs. For example, it was as high as 86.1% in strongly acid spring, while it was as low as 40.3% in simple thermal spring. It was also directly proportional to the number of times that patients bathed in a day. More than 90% of those who bathed over six times per day experienced balneal reaction.<br>According to our survey, generalized symptoms appeared within a week, and there were some cases in which they appeared later than a week. Especially localized symptoms usually appeared 7 to 10 days after the institution of treatment in most cases. It was recognized that there were two types of symptoms of balneal reaction, namely generalized and localized. Generalized symptoms were feeling of fatigue and weariness, increased or decreased appetite, constipation or laxity, lethargy or insomnia, headache, palpitation, dizziness, etc. As for localized symptoms, 75 patients (6.9%) had what was called balneal dermatitis and only 27 (2.5%) showed the symptoms of reactivation of old foci. Dermatitis was mostly observed in those who bathed in acid or sulphur springs, and for this reason it was called sulphur dermatitis.<br>The erythrocyte sedimentation rate became unstable in the early stage of balneal reaction, but it gradually decreased and became stable. The fatiguability progressed in parallel with balneal reaction, but it was temporary in nature. Urobilinogenuria showed most interesting facts; it increased qualitatively and quantitatively in parallel with the severity of balneal reaction and sometimes it appeared prior to balneal reaction. Therefore, we may say that urobilinogenuria is a diagnostic test of balneal reaction. When balneal reaction was studied by means of the liver function test such as Takada reaction, Santosol test and Bromsulfalein test, abnormal results were observed in some cases. But mostly it was not significant. And the author was of opinion that such changes were not indication of dysfunction, but merely due to a stimulation of the liver, because it was transitory and disappeared altogether when curative bathing was discontinued or sometimes even when continued.<br>The various kinds of biological reaction observable in slight balneal reaction was not of high degree, and subsided with the disappearance of symptoms of balneal reaction; they were temporary and reversible in nature. When balneal reaction progressed to such a degree that it produced a sense of sickness, these vital reactions sometimes lost their temporarity and reversibility.<br>From the preceding description, it appeared that so-called “balneal reaction” was wide in range. If the balneal reaction in the broad sense was defined as merely a vital reaction to balneal stimulation, then balneal reaction could be observed in 60 to 90per cent of bathers who bathed five to six times a day in high temperature hot springs. If t