This prospective MRI investigation was performed to investigate septum pellucidum (SP) anomalies in 505 (242 male, 263 female) non-psychotic persons. The mean age of the population was 39.179 +/- 0.904 (40.461 +/- 1.395 male, 38 +/- 1.166 female). There was no significant difference between the means of age in the male and female groups (t-test, DF = 479, p > 0.05). The SP anomalies were classified as cavitation anomalies (Type I) and absence of the SP (Type II). Type I anomalies were subdivided into four groups as isolated cavum septi pellucidi (Ia), cavum septi pellucidi et cavum vergae (Ib), anterior small triangular cavities (Ic), and cysts of the SP (Id). The incidences of the anomalies (Type I + Type II) were 17.31%, 1.89%, 7.55%, 3.53%, 7%, 4.55%, 4.76% and 6.06% for the age groups of 0-9, 10-19, 20-29, 30-39, 40-49, 50-59, 60-69 and 70-79 years respectively. The anomalies were very significantly more frequent in the 0-9 years age group than in the other age groups (chi 2 = 9.7858, DF = 1, p < 0.05). The incidences of the anomalies (Type I + Type II) were 6.34%, 5.37%, 7.22% for the whole, male, and female populations, respectively. These values were 1.39%, 1.65% and 1.14% for Type Ia, 2.77%, 2.89% and 2.66% for Type Ib, and 1.78%, 0.83% and 2.66% for Type Ic. Both Type Id and II anomalies were determined in only one case for each group in females (0.2%). There was no significant difference between the incidences of the anomalies in both sexes (chi 2 = 0.45, DF = 1, p > 0.05).