The international IMPROVE (TM) observational study investigated the safety profile and effectiveness of biphasic insulin aspart 30/70 (BIAsp 30) in the routine treatment of patients with type 2 diabetes. We present analyses for the subgroup of patients who switched from basal insulin to BIAsp 30. Patients in routine care who started insulin therapy with or switched to BIAsp 30 from existing insulin regimens were eligible for this 26-week study. This analysis includes only patients previously treated with basal insulin. Outcomes including adverse events, hypoglycaemic events and glycaemic profile were recorded from patients' notes, recall and diaries. Of the 748 patients included (age 59.7 +/- 11.8 years, diabetes duration 11.4 +/- 7.3 years, baseline HbA(1c) 9.1 +/- 1.6%), 497 were previously using human neutral protamine Hagedorn (NPH) insulin and 245 analogue basal insulin. Overall, major and minor hypoglycaemia rates decreased from baseline to final visit (major: 0.171 to 0.011; minor: 9.70 to 5.89 events/patient-year) and were similar between the subgroups. HbA(1c) and fasting blood glucose were significantly reduced from baseline (NPH prestudy: -1.6%, -2.4 mmol/l; analogue basal prestudy: -1.8%, -2.4 mmol/l), as was postprandial blood glucose, with 33.8% of patients achieving the HbA(1c) target < 7% without hypoglycaemia. Insulin dose increased slightly from prestudy (0.33 +/- 0.21 U/kg), baseline (0.40 +/- 0.20 U/kg) to final visit (0.52 +/- 0.26 U/kg); most patients (76%) followed a twice-daily regimen at final visit. Body weight did not change significantly and treatment satisfaction increased. Patients with type 2 diabetes inadequately controlled on basal insulins may improve their glycaemic control by intensification to BIAsp 30 therapy.