Malnutrition in the early stage has been reported as an independent predictor of survival in amyotrophic lateral sclerosis (ALS). We analyzed retrospectively the effect of variation of body mass index (BMI) on survival in ALS patients. In total, 77 consecutive ALS patients were enrolled from nine hospitals in Japan. Reduction rate of BMI was calculated from BMI before the disease onset and at the time of the first visit to each hospital. We analyzed the correlation between BMI reduction rate and total disease duration. Results showed that the median BMI reduction rate was 2.5 per year (interquartile range 1.3-3.8). The BMI reduction rate was significantly correlated with survival length (p <0.0001). There was also a significant difference in survival between ALS patients with a BMI reduction rate ≥ and < 2.5 (Kaplan-Meier survival analysis and the log-rank test, p < 0.0001; hazard ratio by the Cox model, 2.9816). In conclusion, faster reduction of BMI at the initial stage before the first visit to hospital predicts shorter survival length also in Japanese ALS patients.