STUDY QUESTION Is there a relationship between serum anti-Mullerian hormone (AMH) levels and seasonal variations in serum vitamin D in ovulatory and polycystic ovary syndrome (PCOS) women? STUDY ANSWER Serum AMH levels were not associated with serum vitamin D status even after controlling for relevant co-variants, with this finding being consistent for all causes of infertility. As expected, seasonal variations in serum vitamin D were observed between summer and winter. WHAT IS KNOWN ALREADY AMH plays an important role in maintaining ovarian reserve and modifying follicle sensitivity to FSH stimulation. Studies suggest that vitamin D has the ability to modify AMH production in vitro, yet only one clinical study reports the influence of vitamin D on AMH levels. STUDY DESIGN, SIZE, DURATION This was a retrospective cohort study analyzing the potential interaction of AMH and vitamin D for 340 women (58 PCOS and 282 ovulatory women) aged less than 40 years collected as part of their routine fertility assessment between January and December 2013 at a private fertility clinic in Adelaide, South Australia. PARTICIPANTS/MATERIALS, SETTING, METHODS Patient data including age, BMI, cause of infertility, antral follicle counts (AFC), serum AMH and vitamin D levels, smoking status, and menstrual cycle length for women aged less than 40 years of age, with serum AMH and vitamin D sampled within the same 4-week period were retrieved from a database. The hours of sunlight per day and daily UV index were extracted from a database at the South Australian Bureau of Meteorology, South Australia. Serum vitamin D (25-hydroxyvitamin D) levels were analyzed against seasonal variation in sunlight and UV exposure and serum AMH levels, while controlling for relevant co-variants. MAIN RESULTS AND THE ROLE OF CHANCE Seasonal variations in serum vitamin D were observed between summer and winter (30% variance; P < 0.001), while serum AMH levels (mean ± SEM) remained unaffected by season status (36.9 ± 3.3 versus 38.5 ± 2.7 pmol/l; P > 0.05), even after controlling for relevant co-variants. Overall, no correlation was observed between serum AMH and vitamin D levels, in either the PCOS or ovulatory cohort. Serum vitamin D levels were not significantly related to the underlying cause of infertility (PCOS, diminished ovarian reserve, 'fertile' ovulatory controls). LIMITATIONS, REASONS FOR CAUTION The data used to generate the study findings are cross sectional in nature. While we acknowledge that a longitudinal study monitoring the relationship between serum AMH and vitamin D in individuals over the four seasons would have been ideal, we believe the current findings are robust as our four seasonal groups did not differ for any significant co-variant for serum AMH or vitamin D (age, BMI, PCOS status or AFC) and that there is no significant association between serum vitamin D concentration and AMH production. WIDER IMPLICATIONS OF THE FINDINGS At present, while in vitro studies suggest vitamin D has the potential to modify AMH production, clinical study findings are conflicting. If vitamin D does influence AMH production, this could have important therapeutic implications.