Mass and cystic lesions of the posterior fossa constitute an important chapter in neurosurgical paediatric pathology. Their histological definition is essential for specific treatment. Different types of cysts, can be found besides gliomas, tuberculomas, vascular malformations, radionecrotic lesions or other masses. This paper summarises the results of an observational, descriptive, retrolective study of a group of 30 children with posterior fossa lesions seen from October 1989 to January 1997 at the Hospital Infantil de México (Children's Hospital of México). All the lesions were approached stereotactically to get biopsies and/or to drain cystic lesions. Clinical case notes were reviewed detailing different variables. It should be mentioned that in among these variables, all procedures were carried out using general endotracheal anaesthesia and that in spite of having performed the biopsy or drainage in the CT scanning suite, no infectious processes supervened. Tissue or fluid samples were obtained using the above mentioned procedure although the specimen was insufficient for histological diagnosis in one. For this case, a biopsy done afterwards by direct microsurgical exploration revealed an astrocytoma. In the rest, anaplastic astrocytomas were found in five patients, low grade astrocytomas in 11, and one case of each with the following lesions: medulloblastoma, reactive gliosis, germinoma, teratoma, tuberculoma, bacterial abscess, primitive neuro-ectodermal tumour and chronic leptomeningitis with a decreased cerebellar neuronal population. Two biopsies reported normal cerebellar tissue and in the remaining three, acellular fluid from cystic lesions was obtained. In four patients, cystic drainage was considered part of the treatment. Our results indicate that biopsies of masses and fluid drainage of cystic lesions of the posterior fossa are safe and simple methods. The empirical treatment that is sometimes used for these kinds of lesions, as well as for lesions at any other location in the central nervous system, should be abandoned.