Hypovitaminosis D (<30 ng/ml) is highly prevalent in allogeneic hematopoietic cell transplantation (alloHCT), but the relevance of influencing factors for serum 25-hydroxyvitamin D(3) [25(OH)D(3)] status in adult patients remains unknown. We are the first to have prospectively assessed 25(OH)D(3) status and its influencing factors in 102 patients before and at days +30 and +100 after alloHCT. Among others, we evaluated age, gender, weight, fat mass, season, sun exposure habits, and dietary and supplemental vitamin D intake as factors potentially influencing baseline vitamin D status in uni- and multivariate linear regression analysis. Furthermore, we investigated the impact of changes in fat mass, duration of parenteral nutrition, and acute graft-versus-host disease (aGVHD) on the course of serum 25(OH)D(3). Baseline 25(OH)D(3) concentrations were 16.4 ± 8.9 ng/ml, revealing that the majority (89%) had concentrations beneath the normal range. In multivariate linear regression model, only higher body fat mass remained an independent risk factor for reduced baseline 25(OH)D(3) concentrations (P = 0.007). In the early post-transplant period, 25(OH)D(3) status remained low, revealing a tendency to further deterioration, especially in patients with corticosteroid-treated aGVHD (≥II). Reduced vitamin D status was very common in these patients before and after alloHCT, whereby the most important influencing factors, namely season and dietary factors seem to have little impact. Our findings suggest that monitoring and if necessary, correcting vitamin D status may be indicated at regular intervals before alloHCT and during long-term follow-up. Further investigations of these patients' vitamin D requirements are needed, especially if they are on long-term corticosteroids.