The mean age of the participants was 78.0 years. Significant improvement was evident in each of the variables investigated, as noted in their bladder diaries after one week, compared to before the administration of the medication. The mean night time frequency improved from 3.85 before administration to 1.62 after 52 weeks (p < 0.0001). HUS lengthened from 134 minutes before administration to 250 minutes after 52 weeks (p < 0.0001). Further significant improvement was seen in both the night time frequency and HUS from 12 weeks onwards, compared with one week after the initiation of administration (1 week vs 12 weeks, 24 weeks, 52 weeks: all p < 0.001). There were also significant improvements in the night time urine volume, nocturnal polyuria index, daily urinary frequency, volume of first night time void, IPSS, IPSS-QOL, OABSS, and G8 (all p < 0.05). Significant progress was manifest on the AIS (p < 0.0001), with a positive correlation between the amount of change in HUS and the amount of change on the AIS (p = 0.005, Pearson’s correlation coefficient: 0.325). Furthermore, the greater the change in HUS after administration, the better the PGI-I score (p < 0.0001, Pearson’s correlation coefficient: -0.489), suggesting that both quality of sleep and QOL improved with the administration of desmopressin. The continuation rate was high, with 93.1% at 4 weeks, 89.1% at 12 weeks, 81.2% at 24 weeks, and 71.7% at 52 weeks. Hyponatremia resulted in 5% of the patients discontinuing administration. A low serum Na level prior to administration was found to be an independent predictor in a multivariate analysis of predictive factors for hyponatremia (p < 0.05), and factors such as weight, BMI, and body water content were not significant factors. 16.7% of patients had a PGI-I score of 1, 35.9% patients had a score of 2, 32.0% patients had a score of 3, 15.3% patients had a score of 4, and there were no patients with a score of 5 or more. Patients with a good PGI-I had longer HUS after administration, and there was a significantly higher number of patients with a night time frequency of one void or less (p < 0.05). Examination of effect predictors, with an HUS of three or more hours defined as markedly effective, revealed that the mean night time urine volume was an independent effect predictor (p < 0.05). Muscle mass, body fat mass, and body water content were found not to be predictors.