Introduction: The normal range for platelet counts in children is 150,000 to 450,000 per mm3. Thrombocytosis is defined as a platelet count over 600,000 per mm3 (600 x 10 9/l). They are classified as primary when due to a myeloproliferative disorder and secondary when due to a great number of diseases. Objectives: To estimate the frequency of primary and secondary thrombocytosis in children. To describe the diseases associated with secondary thrombocytosis. To relate the magnitude of the thrombocitosis and the different diagnoses. Patients and methods: 1800 blood counts performed at the Roberto del Rio Hospital between january and december 1998 were analysed, sex, platelet and white blood counts, haematocrit, haemoglobin, MCV and MCHC were evaluated. Results: 584 cases of thrombocytosis were found, representing 32.4% of the blood counts. 334 clinical case notes were reviewed, 62% male. 3 patients 0.9% had a platelet count over 1,000,000, 2 of them had primary thrombocytosis (essential thrombocythaemia and chronic myeloid leukaemia) and the third had a bacterial meningitis. The diseases associated with secondary thombocytosis were infection 48.8% (respiratory 70%), iron deficiency 18.6% tissue damage (burns and surgery) 12.6%. Conclusions: The frequency of primary thrombocytosis is low, when it is less than 1,000,000 a secondary aetiology is most likely.El valor normal de plaquetas varía entre 150 000 y 450 000 x mm3. Se define trombocitosis como un recuento mayor de 600 000. Pueden ser primarias, por un trastorno mieloproliferativo o secundarias a un gran número de patologías. Objetivos: conocer en nuestro medio la frecuencia de ambos tipos de trombocitosis, describir las patologías asociadas y relacionar la magnitud de la trombocitosis con los diferentes diagnósticos. Material y métodos: se analizaron 18 000 hemogramas realizados entre enero y diciembre de 1998, en el Hospital Roberto del Río. Se evaluó sexo, recuento de plaquetas y leucocitos, hematocrito, hemoglobina, VCM, CHCM y diagnósticos. Resultados: se encontró trombocitosis en 584 exámenes (3,24%). Se evaluaron 334 fichas, el 62% eran de sexo masculino. El 0,9% presentó cifras de plaquetas > 1 000 000 x mm3 (dos casos fueron trombocitosis primarias: trombocitemia esencial y leucemia mieloide crónica y un caso de meningitis bacteriana). Las trombocitosis secundarias se asociaron a: infecciones (48,8%), principalmente respiratorias, deficiencia de hierro (18,6%) y daño tisular (12,6%). Conclusiones: la frecuencia de trombocitosis en niños es baja. Cuando la trombocitosis es menor de 1 000 000 x mm3 debe sospecharse una etiología secundaria.