OBJECTIVES: The aim of this study was to determine the frequency of seborrheic dermatitis (SD) by months and seasons in Istanbul, Turkey, and to examine the relationships between SD frequency and meteorological data, such as average temperature (°C), average humidity (%), average sunshine duration, and solar radiation per months. METHODS: The study was conducted at Dermatology Department of Haseki Training and Research Hospital. The data of all adult patients who applied to the dermatology outpatient clinic between June 2018 and May 2020 were obtained. Monthly average climate data values of Istanbul between the same period were requested from the Turkish Ministry of Agriculture and Forestry, General Directorate of Meteorology. RESULTS: During the study period, SD was diagnosed in 5316 patients (5.9% of the outpatient clinic). Among these, 2721 (51.2%) were male, 2595 (48.8%) were female, and mean age was 35.9±15.1 years-old. With respect to monthly distributions, SD frequency was 7.3% in December, 7.1% in February, 6.7% in November, and 6.6% in January; whereas June (3.8%) and July (3.9%) represented the lowest proportion of patients. Frequency of SD was similar in the autumn and winter seasons, despite considerable difference in average temperature. The frequency of SD was found to be strongly and inversely correlated with temperature values (r=-0,646, p<0.001) and monthly maximum humidity value (r=-0.609, p=0.001), while it was also moderately correlated with solar radiation levels (r=-0.442, p=0.027). Minimum or average humidity values and sunshine durations (daily and monthly) were not associated with frequency of SD. Spearman's Rho correlation was calculated to assess the directional relationships between continuous variables. CONCLUSION: Our study shows that environmental factors such as low temperature, low ultraviolet index, and low humidity are critical factors that increase the likelihood of SD development. These data support previous studies in terms of indicating that SD frequency might be increased in climate conditions favoring the growth of Malassezia species.