Background: Long-term follow-up with blood tests is essential for bariatric surgery to be a successful treatment for obesity and related co-morbidities. Adverse effects, deficiencies, and metabolic improvements need to be controlled. Objective: We investigated the effects of time and weight loss on laboratory results in each postoperative phase after laparoscopic Roux-en-Y gastric bypass (LRYGB). Setting: Bariatric center of excellence, general hospital, Netherlands. Methods: We retrospectively evaluated results of 30 blood tests, preoperatively and at 6 months, 1 year, 2 years, and 5 years after LRYGB. The 2019 Dutch bariatric chart was used to define weight loss responses as outstanding (>p[percentile curve]+1 SD), average (p+1 SD to p−1 SD), and poor (<p−1 SD). Results are presented with fifth and 95th percentile cutoff values per blood test for each of these 3 weight loss responses at each of the 4 postoperative time intervals. We used ANOVA to determine mutual relations. Results: Results of 4835 patients were analyzed. Five-year follow-up was 58%. Blood levels of ferritin, mean-corpuscular-volume, thrombocytes, vitamin D, parathyroid-hormone, glycated hemoglobin (HbA1C), triglyceride, total-cholesterol, C-reactive-protein, gamma-glutamyl-transferase, alkaline-phosphatase, creatinine, vitamin B1, and total protein were related with weight loss response. All 30 blood tests were also related with time. For several blood tests, weight loss and time did not only influence median results, but also fifth and 95th percentile cutoff values. Many patients had better vitamin levels after the operation. We observed an increase of parathyroid-hormone and ongoing iron depletion up to 5 years post surgery. Conclusions: Presenting results of 30 routine blood tests, including cutoff values based on fifth and 95th percentile, grouped by weight loss response and postoperative time interval after gastric bypass surgery is new. The elaborate tables and graphs could serve as practical guide for proper interpretation of laboratory results in postbariatric surveillance. Results underline the need for long-term follow-up, including blood tests.