RESUMEN
To find out the nuquency of different clinical conditions causing thrombocytosis in hospitalized patient population. Cross sectional, descriptive study. Combined Military Hospital Attock, 1st Sept 2003 to 1[st] March 2004. All patients referred to haematology department for complete blood counts who had platelet count exceeding 400 x10[9] /L. Out of nine thousand patients, 329 [3.65%] revealed thrombocytosis. The Male to female ratio was 1.4:1, [48%] were adults, ana 52% were children. Platelet count ranged between 403 X10[9]/L and 1365 X10[9]/L In adults, iron deficiency [36%], infections [30%], infections concomitant with iron deficiency [13%], chronic inflammatory disorder [7%], clonal thrombocytosis [5%], haemorrhages [3%]; and in children infections concomitant with iron deficiency [36%], infections [26%], iron deficiency [18%], and acute gastro enteritis [6.5%] were the leading causes of thrombocytosis. Iron deficiency anaemia and infections are leading causes of thrombocytosis in our set up. Clonal thrombocytosis is seen in 5% of adult patients. Relative frequency of different conditions causing thrombocytosis varies with patient age