BACKGROUND: Abnormal sex differentiation and development may present ambiguous genitalia in the newborn or lack of secondary sexual characteristics in puberty. A prompt and accurate diag-nosis should be established to minimize or avoid medical, psychological and social complications. The purpose of this study was to evaluate the causes and clinical characteristics of patients with abnormal sex differentiation and development. METHODS: We analyzed 35 patients with abnormal sex differentiation and development. Twenty patients had been considered or reared as males and fifteen patients as females. The diagnostic evaluation consisted of physical examination, hormonal analysis, sonogram, genitogram, gonadal biopsy and cytogenetics. RESULTS: Among the thirty-five patients, 11 patients were hypogonadism, 9 male pseudoherma-phroditism (5 hypospadia, 2 androgen insensitivity syndrome), 6 female pseudohermaphroditism (4 congenital adrenal hyperplasia), 4 micropenis, 4 congenital anomaly and 1 mixed gonadal dys-genesis. Gonadectomy was performed in patients with androgen insensitivity syndrome and mixed gonadal dysgenesis. Sex of rearing and gender assignment were all concordant with the known sex except one patient, who was previously reared as female and finally reassigned as male due to 5-alpha reductase deficiency. CONCLUSIONS: The causes of abnormal sex differentiation and development were variable. There-fore, an accurate diagnosis should be made by history, physical examination, radiologic and laboratory tests. Proper management and sex assignment are needed in accordance with the cause.