Background/Objectives: Iron replacement and investigation of the underlying disorder are complementary aspects of care. We described diagnoses recorded after intravenous iron treatment and gastrointestinal biopsy findings during follow-up. Methods: This single-center retrospective cohort included 1,164 adults treated during 2015–2025. Linked diagnosis and pathology records were classified relative to the index infusion. Two authors manually reviewed and verified pathology reports. Five age- and sex-adjusted logistic models assessed associations between laboratory values obtained within 30 days before infusion and a gastrointestinal composite, with correction for multiple testing. Results: Among 1,085 adults with observation beyond infusion, median follow-up was 38.2 months. Follow-up diagnosis status was documented in 601 patients; 47 had a diagnosis dated after infusion, including 24 after one year. Across linked sources, 33 patients had post-infusion malignancy records, including 10 with supporting pathology. Post-infusion gastrointestinal biopsies were available for 169 patients. Chronic gastritis was reported in 151 (89.3%), atrophy and/or intestinal metaplasia in 143 (84.6%), and malignant tumors in six (3.6%; 95% confidence interval, 1.6–7.5%). The gastrointestinal composite occurred in 52 patients (30.8%); none of the five laboratory associations met the multiplicity-adjusted threshold. Conclusions: Malignant and nonmalignant diagnoses were documented during follow-up, including beyond the first year after intravenous iron treatment. Linked biopsy reports provided histologic detail in a selected subgroup; the findings do not establish the cause of the original iron deficiency or disease incidence in the treated population.