The purpose of this study was to clarify the relationship of daily hot water bathing at home (DHW) and hot water spa bathing (HSPA) with the number of underlying diseases in middle-aged and elderly ambulatory patients. We defined the number of underlying diseases as the main outcome and dependent (criterion) variable. The frequency and time of DHW and the frequency of HSPA were set as explanatory variables. Multiple logistic regression analysis was performed for each frequency and time, adjusted age and sex. Odds ratio (OR) and 95% confidence interval (95% CI) were calculated. Among the 1261 patients who participated, there was no significant difference in age between males (n = 508, 72.8 ± 6.8 years) and females (n = 753, 73.5 ± 6.9 years). There was also no significant age difference between males (number of diseases: 2.7 ± 2.0 pts.) and females (number of diseases: 2.7 ± 2.1 pts.) in the occurrence of underlying diseases. Frequency and time of DHW were not associated with the occurrence of underlying diseases. However, compared with participants who utilized hot water spa at least once a week, the occurrence of underlying diseases was significantly associated with bathing frequency: one to three times per month (OR 2.72, 95% CI 1.63-4.52); twice or five times a year (OR 1.92, 95% CI 1.25-2.94). In conclusion, lower frequency of HSPA was significantly associated with increased risk of the occurrence of underlying diseases in middle-aged and elderly ambulatory patients. However, the relationship between proactive use of hotwater spa and patients' mental and physical support should be clarified by well-designed cohort studies. The present study was registered as UMIN000033018 by the University Hospital Medical Information Network Clinical Trials Registry (UMIN-CTR) in Japan (refer: https://upload.umin.ac.jp/cgi-open-bin/ctr/ctr_view.cgi?recptno=R000037643)