BACKGROUND: This paper describes trends in hospital activity, hospital admissions, and treatments for colorectal cancer on residents of the South Thames regions (population 8 million) between 1989-1993 against the background of the Calman Report on the future of cancer services in England and Wales. METHODS: The analyses are derived from UK hospital data, which are collected as finished consultant episodes (FCEs). These are defined as episodes "where a patient has completed a period of care under a consultant and is either transferred to another consultant or is discharged." Probability matching was used to derive patient-based records, matching FCEs to admissions. A total of 18,542 South Thames residents aged 40-99 were admitted for colorectal cancer between 1 January 1989 and 31 December 1993. Time trends were analysed for procedures, FCEs, admissions, and patient numbers by admission type (ordinary admissions and day case admissions). RESULTS: Between 1989 and 1993 inclusive colorectal cancer admissions doubled (98% increase p (trend) < 0.0001). These admissions were a result of a 6.4-fold increase in day case admissions and a 41% increase in ordinary admissions. The proportion of patients having a day case admission rose from 9% in 1989 to 18% in 1993 (p < 0.0001). Overall, 2894 (16%) patients had a day case admission; 1894 of these (65%) were also admitted as ordinary admissions. The number of FCEs and admissions per patient rose from 1.37 and 1.28 respectively in 1989 to 2.09 and 1.99 respectively in 1993. FCEs were between 5% and 8% higher than admissions over the five years. The number of ordinary (that is, overnight) inpatient admissions per patient rose from 1.23 to 1.41 over the five year period and day case inpatient admissions from 1.25 to 3.45. Chemotherapy accounted for 50% of the rise in day case admissions; colonoscopy and sigmoidoscopy were associated with a further 18%. Fourteen per cent of the increase in ordinary admissions was also because of chemotherapy. CONCLUSION: The monitoring of site specific trends in admission, treatments, and procedures on a population basis should be a core requirement of health authorities to inform needs assessment, resource allocation, and service planning. The rise in admissions and chemotherapy treatments have implications for drug costs, laboratory and inpatient services, monitoring, and clinical audit.